This study was designed (1) to determine differences in classificatory skills of low socioeconomic status children under intervention and non-intervention conditions; (2) to determine differences in classification skills of bilingual, biliterate, and monolingual students; and (3) to study differences in classificatory skills as a function of age. A total of 90 Mexican-American and Caucasian kindergarten, first grade, and second grade children were the subjects. Half of the children participated in an intervention program designed to develop classification skills. Instruction was conducted in groups of five bilingual, five biliterate, and five monolingual children in each of the three age groups. The findings indicate that the intervention group made significant gains over the control group in two modes (categorical-inferential and relational). There were no significant age differences between language groups, but significant age differences were present in the categorical-inferential and descriptive modes. The findings are discussed in terms of treatment, language, and age. Observations of pupil behavior during testing are also discussed. (SDH)