Students in an introductory biology course had laboratory sessions in one of two patterns: as one three-hour session per week, or as two one-and-one-half hour sessions, one after each lecture. Each of the twelve instructors involved taught one section using each pattern. The Nelson Biology Test, the Watson-Glaser Critical Thinking Appraisal, and a content examination were administered at the beginning and end of the semester. The group with the two laboratory sessions per week was significantly better at applying biological knowledge in problem solving, but there were no significant differences between the groups with respect to critical thinking ability and attainment of biological knowledge. There was a significant teacher effect and an interaction between teacher and scheduling pattern in terms of biological knowledge attainment, but not in the other variables. (Author/AL)