A summer box-office boom for the gentle, humane fantasy

Variety, the show-business newspaper, sums it up in a headline: ''Film B. O. Is 'Extra-Terrestrial' ''!

It isn't only ''E.T.'' that's burning up the box office, of course. Other pictures are packing in moviegoers, too - especially ''Rocky III'' and ''Firefox ,'' with ''Star Trek/The Wrath of Khan'' and ''Poltergeist'' also in the running. ''Annie'' and ''Blade Runner'' are close behind.

But the phenomenon of the year is ''E.T.,'' which pulled in nearly $3.5 million each day during June. That makes it ''the fastest draw in world film history,'' according to A. D. Murphy, a movie-business analyst. And there's now speculation about this friendly film's pulling past ''Star Wars'' to become the No. 1 hit of all time.

Considering the current box office in general, it's hard to remember there was a bad slump as recently as January. In the period from June 1 to July 4, says Variety, viewers paid more than $500 million to buy more than 150 million tickets. Nobody is predicting this pace will keep up. But it's pretty certain 1982 will set a new record for summer-movie earnings.

Beyond the awesome figures themselves - such as ''E.T.'' making more than $ 100 million in just 30 days - it's interesting to note what spectators are choosing to see. Summer is always a big time for fantasy, for example, and this year has supplied plenty of same. By whopping margins, audiences are opting for the gentler and more humanistic entries, including ''E.T.'' and the ''Star Trek'' film, and rejecting the vicious violence of ''The Thing,'' which is performing poorly despite the hype that surrounded its release.

As for ''Tron,'' the heralded science-fiction epic from Walt Disney Productions, it has opened disappointingly, attracting sizable crowds in big cities but flopping in smaller towns. By contrast, ''Poltergeist'' has earned bushels of money. And ''Blade Runner,'' although a violent film, has been doing well, but it is still a comparative newcomer and could fall off if audiences continue to reject gory fare.

The other smasheroo of the summer has been ''Rocky III,'' which reminds us once again that nothing succeeds like success. ''Rocky III'' is an instant replay of ''Rocky II,'' with the same prizefight story, the same corny characters, and the same knack for wrapping a crowd - or a mob - around its little finger.

There's just as much muscle and just as little subtlety in the new Clint Eastwood epic, ''Firefox,'' which may account for its success. Meanwhile, the expensive ''Annie'' is holding its own (though not exactly thriving) by attracting an audience largely of little girls and their parents. It's a living.

In sum, the movies may not be better than ever, but they are earning more than ever. And it's encouraging that a picture as warm and upbeat as ''E.T.'' is leading the pack, while less heartfelt entries fall by the wayside.