A critical thinking assessment was used to compare biology students in a nonmajors course who participated in either a multisemester research program or in a one-semester lab study. Initially, it was thought the former would be more effective in promoting critical thinking improvement than the latter. Data from nine biology classes with pre- and posttests were used. Six sections participated in a multisemester fly genetics study, and three sections did a one-semester corn genetics study. Each student wrote a draft and a final report. In the former study, biology students studied Drosophila eye color mutants and wild type that were crossed through parental, F[subscript 1], and F[subscript 2] generations. In the latter study, parental, F[subscript 1], and F[subscript 2] ears of corn were examined. The students in the corn laboratory group actually did better on the posttest. Reasons for this result and other comparisons suggesting the value of a short, multiple-choice assessments are discussed.