The associations among second- and third-grade students' content-area knowledge, vocabulary, and reading gains and the science instruction they received were examined in this exploratory longitudinal study. We also asked whether there were child characteristics x instruction interaction effects on students' content-area literacy. Second graders (n = 88) were followed into third grade (n = 73). Classrooms were observed all day in the fall, winter, and spring, and amounts and types of science instruction and language arts instruction were recorded. Results revealed that specific types of science instruction were related to second- and third-grade students' gains in content-area literacy skills and that the relation of science instruction to outcomes depended on students' fall content-area knowledge, vocabulary, and reading skills. These results suggest that science instruction may promote students' developing content-area literacy growth and may be more effective when it is implemented taking individual student differences into account. (Contains 3 figures, 8 tables, and 2 notes.)