Chemistry students appear to bring significantly higher critical-thinking skill to their nonmajors course than do biology students. Knowing student preconceptions and thinking ability is essential to learning growth and effective teaching. Of the factors investigated, ethnicity and high school physics had the largest impact on critical-thinking skill differences. Years of high school science, GPA, parents' education, and household income, commonly believed to influence academic performance, did not affect critical-thinking differences in this study. These factors should be considered when designing and assessing nonmajors courses in order to maximize learning. (Contains 5 tables and 1 figure.)