When William Casari moved to the Grand Concourse seven years ago, some people passing him on the sidewalk figured he was either an undercover police officer or a Yankees fan who had wandered off from the stadium.

“Several times I would hear as I walked by, ‘Is there a Yankee game today?’ ” said Mr. Casari, a 48-year-old archivist at Hostos Community College. “I didn’t personally feel uncomfortable, but people mistook me for a stranger in the neighborhood.”

The reason for their suspicions, he guessed, was that a white face like his was still a rarity among residents of the Concourse neighborhood in the South Bronx. That had been true for 40 years, ever since an exodus that cost the Bronx over 300,000 residents and turned the Concourse into an emblem of white flight and urban disenchantment.

But now Mr. Casari is far less of a curiosity. More middle-class professionals, many of them white, are joining him, buying co-ops with sunken living rooms and wraparound windows for under $300,000 in Art Deco buildings that straddle a boulevard designed to emulate the Champs-Élysées.