''I'm the only person with X.P. in North Carolina,'' explains a lively 14-year-old girl named Alixe Johnson. ''Coming here is like returning to normality. At home, I'm under constant surveillance because my mom is terrified I'll do something silly . . . like go out into the daylight.'' Alixe hates being an outsider; she refuses to wear the scarves and other coverup garments that the younger kids wear. ''The thing is, what am I going to do when I need to drive a car?'' she says. ''You can't exist in North Carolina without a car.''

Although automobile windows can be tinted to screen out UV rays, how does an X.P. sufferer safely get to and from a car? Even just a few minutes of exposure can cause a third-degree burn. It's a vexing problem, one that most of us never have to think about. Alixe shrugs gloomily. ''I wonder if you can even drive in this country if you have X.P.,'' she says. The thought seems to bother her more than the excisions of the four precancerous lesions she has already endured. (Overhearing this, Collin suddenly pipes up: ''Four? I've had 30!'')

As some of the younger kids head upstairs to bed, the teenage campers stay behind in the deserted ballroom to play guitars and sing Simon and Garfunkel songs. Sitting together on the floor, boys nudge innocently against girls. Meanwhile, first light begins to creep ominously into the hotel foyer upstairs. Having spent the entire night running skittishly up and down stairs, huddling in giggling groups and exchanging gossip, the X.P. kids appear exactly like any others, except that they are underground and are counting the critical minutes to daybreak. For them, the morning sun is something to dread.

''You can't let dawn sneak up on you,'' one quips a little sadly.

''You know, there's one thing that's a drag about having X.P.,'' Heather drawls sleepily.

''What's that?'' I say.

''I'd really love to know what it's like to have a tan.''

Katie Mahar lives with her family in a restored 19th-century Colonial farmhouse set a quarter-mile from the Taconic State Parkway. At first sight, the house, which rests atop a wooded slope, seems unremarkable. Only as you are ringing the doorbell do you notice that the windows are tinted as darkly as those of a Mafia sedan. Inside, you feel as if you are in a twilight world. The beautiful 1860's interior seems to have never been touched by ordinary daylight. Through windowpanes covered with a chemical coating that resembles Kodak film, a visitor can see a dimmed outer world of trees and sky tinged with musty gray. This is the outside world as Katie sees it. That is, on the rare occasions that she is allowed to part the curtains and blinds at dusk, peeping out at the day's last lethal moments of sunlight.

Katie has two older brothers and an older sister, all of whom are healthy. But because of the odd hours she must keep, she spends a lot of time playing by herself. Camp Sundown, alas, takes place only a few weeks each year. Fortunately, Katie is a self-sufficient child. When I visit her at home one crisp evening, I find her zipping joyously around her backyard. She will often stay outside like this for hours, her mother tells me later, long after the other children in the neighborhood have gone to bed.

As we tramp around the woods out back, Katie shows me the places she knows by heart: a birdhouse, a gully, a constellation of trees. ''Did you know that turtles lay their eggs at night?'' she asks excitedly. ''Oh, yes. There's even a butterfly that you can only see at night.'' It is clear that her parents have taught her such facts in an effort to make her feel less isolated.