As hypothesized, participants' English proficiency developed more slowly in schools in which student bodies had relatively high percentages of native speakers of Spanish, faster in schools in which student bodies had relatively high percentages of monolingual native speakers of English (p < .03). Although participants' English-proficiency raw scores generally increased substantially between successive longitudinal occasions, the grade- level percentile ranks (derived from U.S. norms for native speakers of English) for these scores increased very little--a probably frustrating contrast between absolute and relative achievement. Participants who returned to schools in Puerto Rico continued to develop their English-language proficiency, although considerably more slowly than during their stay in stateside schools.