This paper presents the results of a study which examined heritability ratios for the major ethnic and socioeconomic groups in the Los Angeles Unified School District. Scores on three measures (the Cooperative Primary Reading Test, the Comprehensive Test of Basic Skills, and a composite set of intelligence tests) were compared for twins from different populations (upper-middle and lower socioeconomic status black, white, and Spanish-surname). On all of the measures there were significant differences for ethnicity and socioeconomic status. Heritability ratios varied for each population and measure. It was concluded that extrapolating from data on white middle class twins is unwarranted and that for other populations heritability ratios are much lower. (Author/RH)