To better understand the dynamics of online student test taking, including the likelihood of cheating by large numbers of students, we examined test-taking patterns and outcomes of weekly online quizzes in two large undergraduate biology lecture courses. Students taking a quiz late in a 1-3-day quiz access period performed 10-15% worse on quizzes than the students who completed the quiz early. Quiz access time was also negatively correlated with performance in other course components and course grades. These patterns suggest that academic dishonesty was not a determinant in unsupervised online quiz performance. Students generally completed quizzes in late afternoon or evening hours, but students who completed quizzes between midnight and 8 a.m. had significantly lower quiz grades than their peers. In addition, upper-division students were more likely to characterize weekly online quizzes as more helpful for their learning than the lower-division students. (Contains 5 figures and 3 tables.)