Aulnay-sous-Bois, France — These days, many things are changing in this working-class suburb of some 76, 000 just north of Paris. Municipal subscriptions to L'Humanite, the Communist Party newspaper, are being canceled. Several hundred party members are being discreetly asked to leave their town jobs. Even Avenue Salvador Allende may soon be renamed.
The transformation in Aulnay symbolizes the present difficulties of the French Communist Party. Not long ago, neo-Gaullist Jean-Claude Abrioux defeated incumbent Communist Pierre Thomas in a hotly contested mayoral by-election, ending 17 years of party rule here.
''Every day we discover another thing the Communists have done that we want to change,'' says Mr. Abrioux. ''It will take a long time to root out. But we will rid this town of Marxism.''
Aulnay is not an isolated case, either. Communists were accused of stuffing the ballot boxes in March's elections in seven towns of the so-called ''Red belt'' of Paris suburbs that have long been party fiefs.
So far, four by-elections have been held. The Communists have lost each one. Two more are scheduled in the coming weeks.
In all, the party has lost 25 major municipalities to the opposition this year, leaving it with only 57 of the 72 towns it controlled in March 1983. From a high of 28 percent of the vote in elections after World War II, the party fell to 15 percent in the 1981 presidental contest, and recent polls show further erosion. Communist support may be as low as 10 percent.
''We may be witnessing the permanent decline of the French Communist Party,'' asserts noted political analyst Alfred Grosser. ''With these elections, it's become a weekly spectacle.''
The setbacks have put the governing Socialist-Communist alliance under new strain. A worried party has been increasing its criticism of the government, and speculation is growing that the Communists may soon leave the coalition - or be thrown out.
The party faces a tough choice. Participation in the government gives the Communists control of four ministries and important posts throughout the bureaucracy - in short, respectability. Moreover, breaking with the Socialists would leave them open to blame that they were deserting the left during its crucial hour.
Such criticism hurt the Communists badly after they split the left's alliance in 1977.
But by staying in the government, the Communists are forced to accept the unpopular Socialist bear hug. President Francois Mitterrand's plan to trim money-losing heavy industries strikes at the party's working-class base.
At Aulnay, for example, the government has let Citroen trim the work force at its large plant. This reversed election promises to increase employment in the automobile industry, and local Communists blame such decisions for their defeat.
''We lost primarily because the people are dissatisfied with the change on the national level,'' says ex-Mayor Thomas. ''They are upset because promises that were made have not been implemented.''
Still, the setback in Aulnay shows there is more to the Communists' decline than dissatisfaction with the Mitterrand government. Suburbs such as Aulnay have been working class for generations. In the 1920s, this was where the new Communist Party had its first successes, and soon, the Communists had constructed their ''Red belt'' stretching around most of the capital.
Postwar expansion turned the suburbs into blocks of factories and drab slabs of cheap public-housing projects. Communism's roots grew deeper - and the party set an example of good local government.
In Aulnay, a sports stadium was built and a myriad of athletic teams organized. A cultural center was constructed, and the town has a full schedule of plays and concerts. Even a public library, a rarity in France, was funded.
''The Communists succeeded in installing in their towns exemplary social management,'' says Patrick Jarreau, Le Monde's specialist on Communist affairs.
Communist-hater Abrioux does not deny this. ''The stadium, the cultural center - how could you not like those things?'' he asks.
But recently, Communist management has been called into question. Sociologist Raymond Pronier has written a book in which he alleges the party has used municipal funds to pay the salaries of party activists and to finance contracts for party-owned companies.
Abrioux made Communist dishonesty a key part of his electoral platform. He constantly reminded voters of the confirmed electoral fraud that originally denied him victory, while publicizing other alleged abuses such as the L'Humanite subscriptions and the status of some 300 municipal workers whom he charges were hired to do party, not town, work.
''All were party militants,'' he says. ''Many spent their time writing propaganda.''
A demographic mutation also finally caught up with the Communists. French workers are becoming better off. They can afford better lodging than the depressing public-housing blocks in Aulnay. Some have bought houses in Aulnay, but many have bought homes farther from Paris.
The new homeowners don't like the high taxes the Communists collected for their public projects. They don't like the North African immigrants who have moved into the town's housing projects either. In short, they no longer identify with the Communists.
''I used to vote for the Communists,'' construction worker Roger le Previer says. ''But I changed this time. The taxes on my house are too high. The town's going downhill socially. What did the Communists do about it? They were corrupt.''
Despite all these problems, no one in France is counting the Communists out just yet. As layoffs continue, many workers may turn back to the party and its powerful trade union, the Confederation du Travail, for solace. If the Communists leave the government, they might pick up new followers among the unemployed.
Moreover, the party could renew its image by replacing discredited party chief Georges Marchais with a younger, more articulate leader. Transportation Minister Charles Fiterman, the party's No. 2, could be such a man.
''This could be a terminal decline for the Communists,'' says Le Monde's Jarreau. ''Or it could be just a bad moment for them.''
In Aulnay, no leadership changes are planned. But Thomas says he is confident he can win back the town hall in the next election five years from now.
''We have a good record,'' he says. ''The right won't be able to equal it, and when the public sees that, we will regain their confidence.''