Sixteenth and Mission Streets, as busy as rush hour, is still the heart of the heart of the city's Spanish-speaking community. The fruit stands are still piled shoulder high with plantanos, yucca and mangoes, the saleswomen in the discount department stores still greet customers with ''Hola'' and families still wear their Sunday best to attend services at the Pentecostal Iglesia de JesuCristo.

Sixteenth and Valencia Streets, just one traffic light away, is another story. The people sipping lattes at the new Intermission Cafe are young, trendy and non-Hispanic. The shops -- from vintage clothing stores to bars to used-book stores -- cater to the same. Every weekend, the bars along Valencia draw upwardly mobile hipsters from all over the San Francisco Bay Area. They jam into the pubs and clubs, zigzagging into the streets like lines of dominoes.

But Valencia Street is not the only pocket of change here. Madre y padre grocery stores now stand side by side with health food stores, auto shops with art galleries. The entire Mission District, port of entry for San Francisco's Hispanic immigrants for more than 50 years, is changing by the day. New people, people who have money, are moving in altering life for everyone. Sagging Victorian houses that landlords had chopped into two or three rental units are sold for a half-million dollars, and warehouses are becoming loft condominiums in the $300,000 to $400,000 range. The neighborhood, in short, is gentrifying.

More and more, people here worry that these changes have come at a heavy cost to the Mission's working-class residents. For all its grit, the Mission has played an important role in a city where prices were already extraordinarily high and low-income housing especially scarce. It has been a cultural center for Hispanic people in the Bay Area, the one neighborhood where new immigrants knew they could find a home. Now, there is a fear that as San Francisco becomes more affluent, the ingredients that made the Mission District unique will be lost.