LAST WEEK I WAS in California; the week before the Alabama and in New York. In all three places, Canada's Mariposa Folk Festival was a steady topic of conversation. Who's appearing this year? How do you get there? Do you need a passport?

Making its ninth annual appearance this weekend, Mariposa is as much a part of the North American folk scene as saggy blue jeans, out of tune guitars, and worn out cowboy boots. It is now firmly entrenched as one of the North American folk festivals, one of the very few which can be counted on to draw large crowds of people to a well-planned weekend of informal workshop gatherings and entertaining evening concerts.

Because of it first-class internal organization and a particularly well-run show last year, Mariposa has been able to buck the waning interest in folk (rock, of course, borrowed heavily from the folk scene and took many folk fans with it) and survive as a financially and artistically successful Festival.

Mariposa is a groovy place to be, in the eyes of folksters. They come from near and far to play for what is really only a pittance compared with the massive fees they can command on the concert circuit.

The truth of that observation was brought home to me last week as I sat poolside at the home of Stephen Sills, high in the secluded Laurel Canyon behind Los Angeles. There was a crowd of us gathered around, hamburgers and cokes in hand, people like Gracie Slick of the Jefferson Airplane, Joni Mitchell, David Crosby (formerly of the Byrds), Graham Nash (an ex-Hollie), Stills and Neil Young, son of Globe and Mail columnist Scott Young.

Joni, who'd just returned from Stratford, was talking about the Canadian folk scene, and Young seemed a trifle dismayed. Several months ago, after the Buffalo Springfield had shot itself to death and Young was aspiring to solo folk heights, Mariposa organizers had invited him to appear. He accepted, then, a few weeks ago, he was asked to join Crosby, Stills and Nash, a tremendously successful West Coast pop conglomerate, thereby making it Crosby, Stills, Nash and Young.

He also accepted that offer, then found that because of recording commitments it would be difficult to leave the group for the Mariposa that he would have to pass, and organizers subsequently booked in his place a young Ottawa folk singer, Bruce Cockburn.

Last week, as we sat by the pool, Young decided that he desperately wanted to make the Mariposa scene. When he called, however, he found that the schedule was full to overflowing. He and his wife planned to come anyway, just to be there, and Stills was thinking of joining him.

"I really want to be there," Young was saying. "I've lived in Los Angeles for three years and I hate the place."

Young's joining the Crosby, Stills and Nash triumvirate came as a surprise to most observers. After the downfall of the Springfield, he'd announced that the group scene just wasn't for him. But that was before Crosby, Stills and Nash banded together and demonstrated that beautiful music-flowing with the richness of the best folk music - can be made without too much compromise.

Young will, however, continue as a solo star, in addition to playing guitar, organ, singing, and writing with C.S.N. and Y. He has a three year commitment with Reprise Records to produce two solo albums per year. "I've got to be able to go the solo route occasionally, because it's an important part of me.

"When I was with the Springfield, I held back. I was paranoid about my voice. So on my first solo LP, I buried my voice intentionally. The second LOP I brought it up more. I had more confidence. That's what working with Crazy Horse had done. (Crazy Horse is his new backing group). It's given me more confidence. That's why I want to continue as a single. But I won't be singing that much with the new group. Mostly just playing a lot."