This year's great debate about immigration has featured, in certain locales, a small side argument that turns on something basic: food. Immigrants are flocking to street vendors whose carts are laden with pupusas, falafel and other familiar goodies from home. Those carts, in turn, have engendered complaints about health, traffic obstruction and unfair competition.

"We have maybe 30 to 40 mobile vendors on any given day--probably more on the weekend," says Will Campos, a councilman in Prince George's County, Maryland, describing the polyglot suburb of Langley Park. Campos thinks that's too many, so he has proposed limiting the number to eight vendors in residential areas and seven in commercial districts.

That sounds like quite a cutback, until you realize that the county currently bans such vendors altogether. They're all illegal. That hasn't stopped them. The only practical way to control the problem, Campos argues, is to lay down reasonable limits and hope that the vendors who lucky enough to get permits will help crack down on cheaters.

But strict limits may not work any better than an outright ban. Cities and counties that stiffen their resistance to street vendors find that tougher regulations are hard to enforce if there's a market to be catered to. Brevard County, Florida, for instance, issues licenses for roadside stands but finds that unlicensed vendors still turn up after hours. "They soon learn that we're out 8 to 5 Monday through Friday, so you're going to see them mostly out on Saturdays," says Paul Marion, the county's deputy tax collector.

Street vendors can offer more than quick purchases. They fill seasonal gaps and step in when there are fairs or festivals or other events for which building permanent establishments makes no practical sense. Officials in Phoenix are actually trying to encourage more cart vendors downtown, in hopes that they will add a little life to the city's sluggish streets.

Because the vendors serve so many purposes, they're not going to go away through fiat. Weighing their benefits against the perceived seriousness of the problem and the difficulties of control is a task that is bound to produce different answers in different places. "Vending," says Regina Austin, a law professor at the University of Pennsylvania, "is right up against that soft line between the formal and the informal economy.