Properties of maintenance and loss of Spanish/English bilingualism were investigated in 308 high school students of Mexican background. Key variables investigated included: (1) actual and self-reported proficiencies in Spanish and English; (2) self-reported language choice behavior in various settings; and (3) language attitude. The biggest difference in Spanish proficiency was found between the student born in the United States of parents born in Mexico, and the student whose parents were born in the United States. Maintenance of Spanish proficiency was associated mainly with adult language practice in the home, and was not predicted by the subjects' choice of language outside the home or by language attitude. In turn, adult language choice was found to be affected by the demographic fact of immigration, the adult's ability to use English in the home, and increasing distance in the familial social ties to Mexico. Outside the home, language choice showed rapid and constant shift toward English, unrelated to Spanish proficiency but predicted by the subjects' language attitudes. Language attitude also appeared to contaminate self-reported proficiency in both Spanish and English. Finally, testing suggests that attrition of Spanish is best characterized as difficulty of retrieval rather than total loss. (Author/MSE)