The virtual laboratories developed by a life sciences department at a public university in the US were designed for use by college students enrolled in an introductory biology course. The results analyses conducted to examine their effectiveness indicated that self-selected users of the virtual laboratories outperformed non-users on laboratory practical exams and on items relevant to the laboratories included on the final practical exam but not on the final exam or other tests. A second comparison of two groups of users and non-users, matched by their results on the first hourly exam confirmed the findings. The finding that the users were more successful than the non-users could not be supported by a conclusion that the students had initial differences in quality. The results were replicated in a second semester, virtual laboratories-relevant final exam items, but not on the final exam or other tests. Users expressed positive attitudes toward the virtual laboratories. (Contains 6 tables.)