This study investigates the way collaborative learning that occurs primarily outside the classroom affects college students' understanding of science. Collaborative learning is particularly important for the increasing number of nontraditional students who have limited time available for study groups and other peer learning activities occurring outside of class time. Using a national study of 4,644 college students of various academic majors, multiple linear regression was used to identify variables that enhance science learning. Time spent in peer learning settings, such as teaching science to peers and discussing science with peers, were the strongest predictors of understanding science; moreover, this finding was consistent even for nontraditional students who reported less frequency of engagement in such activities. The study suggests that science educators can enhance learning when they structure their courses to include peer learning that engages students with each other over science issues outside the classroom.