In a split second, standing on the concrete steps outside her sister’s Washington, D.C., apartment, the African-American 15-year-old decided to try being a big girl. She took her first puffs.

By June, she had figured out how to inhale and was buying her own packs of Newports.

But smoking four or five cigarettes a day, Crowder found she had less stamina for swimming and basketball. She didn’t like the smell. Besides, the boys she knows prefer girls who don’t smoke. They be like, ‘Don’t be a draggin’ lady.’ By July, she had quit.

The briefness of her flirtation with cigarettes is part of a phenomenon that has deeply encouraged and yet deeply perplexed researchers and policy-makers as the Clinton administration sets out to curb tobacco use among young people.

The percentage of black teen-agers who smoke cigarettes on a regular basis has plummeted in recent years, according to a variety of national studies. There has been no such decline among their white peers. The net effect is a wide gap in the popularity of cigarettes among youths of different races.

Last year, 5 percent of black U.S. high-school seniors reported that they smoked daily, compared with 23 percent of their white classmates, according to an annual survey conducted by University of Michigan researchers.