A study of bilingual development in preschool children had two components: (1) a followup of a previous study in which the English and Spanish language development of children in a high-quality bilingual preschool remained stable over time, and (2) a replication of the study with a different cohort. The original study found that native Spanish-speaking children in a bilingual preschool program and children not in the program had improved English and Spanish skills after one year, but the bilingual program participants had especially large gains in English. Followup showed that both groups continued significant gains in both languages during a second year, with the bilingual program children continuing to make larger gains in English. The replication sample was a similar number of children from the same community, similarly divided into bilingual program and control groups. Similar results emerged from this study. It is concluded that participation in a high-quality bilingual preschool program does promote development of both native and second language skills, does not impeded native language development, and significantly enhances second language learning. (Contains 12 references. Appended materials include charts and graphs of the two studies' findings.) (MSE)