This study examined the influence of cognitive ability and student activities on high-school students’ science achievement. Students (n=1651) from four high schools in three states were assessed in terms of their cognitive abilities (i.e., science knowledge, reading skill, and metacognitive reading strategies), course involvement, reading interest, and TV habits. Science achievement was measured in terms of students’ course grade, comprehension of a science passage, and performance on a statewide standards of learning (SOL) test. Course involvement significantly predicted only course grade, whereas reading interest predicted SOL scores and science passage comprehension. Cognitive abilities and TV habits predicted all three of the student achievement measures. However, the effects of these cognitive variables interacted in interesting ways.