Bilingual vs. unilingual instruction was studied in the teaching of 5 positional-preposition concepts (e.g., under) to Mexican American Headstart 4-year-olds. Treatments consisted of instruction in Spanish only, English only, or bilingually to the 48 subjects exposed to the prepositional concepts via tape-recorded programmed instruction given daily for 10 minutes. Assignment to control and treatment groups was randomly stratified by sex, and subjects were pretested the first day of instruction and posttested on the final (3rd) day. These tests, administered in English and Spanish, included a transfer test consisting of a 2-dimensional presentation of the same prepositions and a learning task in the language opposite from instruction. All criterion tests required verbal labeling and demonstration. Analysis of covariance and Newman-Keuls comparisons indicated that posttest scores of the treatment groups were higher than those of the control group, reflecting the effect of instruction vs. no instruction. Significant differences were found as a function of language exposure in the home, but no significant differences were found between treatment groups. The study is limited by sampling practices and restricted treatment time. An appendix contains the instructional treatments used. (Author/NQ)