Oh, so that's why they hadn't done this before.

For years people have wondered why comic actors Robin Williams and Billy Crystal have never been on-screen together in a film, especially in light of some of their memorable "Comic Relief" appearances together. Well, "Fathers' Day," their first feature-length pairing, answers that question — they don't play particularly well off each other in large doses.

In this surprisingly vulgar comedy, Williams steps out of character as often as he did in the animated "Aladdin" (he was supposed to in that film), while Crystal stands back, almost anticipating Williams' next humorous move.

Also, the film is dishonestly marketed as a "family comedy," and although it is very funny in places, unless your idea of "family humor" involves gags about vomiting, human waste and anatomical parts, you may be better off skipping it.

Based on "Les Comperes," a French comedy, "Fathers' Day" is a sort of buddy road-picture, with Williams and Crystal teaming up to find a teenager (Charlie Hofheimer) whom each believes to be his son.

Each is told by Colette (Nastassja Kinski), a former flame of both men, that he could be the father of Scott (Hofheimer), a rebellious teen headed for San Francisco. So, Jack Lawrence (Crystal), a successful L.A. attorney, and Dale Putley (Williams), a suicidal would-be writer, head off separately for the Bay area.

Initially thinking each is after a different person, the two men combine forces to track down a group of teens following real-life rock act Sugar Ray (a band that doesn't exactly inspire that kind of loyalty in the real world).

Unbeknownst to the duo, the boy is also being pursued by drug dealers, from whom he stole $5,000. And Jack is being trailed by his wife, Carrie (Julia Louis-Dreyfus, from TV's "Seinfeld"), who mistakenly believes he and Dale are having some sort of kinky relationship.

Predictably, things come to a head when the "fathers" finally catch up with Scott, as do the criminals. And eventually, the truth about the boy's parentage finally comes out (in an even more predictable development).

Besides the pedestrian nature of the writing and crudeness of its humor (veteran screenwriters Lowell Ganz and Babaloo Mandel should have known better than to "dumb" the material down this far), the film has a huge problem — its "heroes" are often unsympathetic.

Dale's mood swings are as irritating as they are humorous, while Jack's violent nature (there are many scenes of Crystal head-butting other characters) is simply too vicious.

Even more out of place is the cruel treatment of Colette's husband (Bruce Greenwood), whose only "sin" seems to be that he's a bit reserved in his affections for Scott and who has one painfully unfunny misadventure after another.

"Fathers' Day" earns its PG-13 rating for the aforementioned vulgar jokes and references, some violence (Crystal's head-butting routine) and a few profanities.