This study compared the relationships of self-efficacy and reasoning ability to achievement in introductory college biology. Based on the hypothesis that developing formal and postformal reasoning ability is a primary factor influencing self-efficacy, a significant positive correlation was predicted between reasoning ability and degree of self-efficacy to complete biological tasks. Further, reasoning ability was predicted to be more highly correlated with course achievement than self-efficacy. The study involved pre- and posttesting 459 introductory biology students. Both self-efficacy and reasoning ability increased during the semester. As predicted, self-efficacy and reasoning ability were positively correlated. Depending on the nature of the achievement measure, reasoning ability accounted for some 15 to 30 times more variance in achievement than self-efficacy. Also, as predicted, reasoning ability was a strong predictor of self-efficacy, but self-efficacy was not a strong predictor of reasoning ability. Self-efficacy estimates and achievement were higher for the concrete tasks than for the formal tasks and higher for the formal tasks than for the postformal tasks. In general, students tended to overestimate their abilities to carry out the concrete, formal, and postformal tasks. Results support the study's working hypothesis that intellectual development continues for some students during the college years, that a postformal level of intellectual development exists, and that reasoning ability is a primary factor influencing both self-efficacy and achievement. Student overestimation of their abilities may contribute to complacency, lack of effort, and to less than optimal achievement. Consequently, it may be advantageous early in the semester to provide students with particularly challenging tasks that "shock" them out of their complacency and perhaps increase their effort, their reasoning skills, and their achievement. (Contains 1 figure and 6 tables.)