This research study analyzed the growth of the cognitive critical thinking skills of college calculus students from six sections of Calculus 165 at Iowa State University. For one semester subjects from three experimental sections learned calculus in a consistently facilitated active learning environment. Subjects from three comparison sections learned the same material in a traditional passive learning environment. Fidelity of treatment--i.e. adherence to an active learning format, was verified by direct observations. The main difference between any active and passive learning environment is the amount of time spent on instructor activities (passive learning) versus the amount of time spent on student activities (active learning). Results from two separate observation sessions for sections from both the treatment and the comparison group, clearly showed a dramatic difference in the type of learning climate that existed;At the beginning of the semester the students were informed of the opportunity to be a part of the study. During the second week of the semester subjects from both the experimental and the comparison group took Form A of The California Critical Thinking Skills Test: College Level (CCTST) as a pretest. During the last week in the semester subjects from both groups took Form B of the CCTST as a post test;Analysis of paired statistics for the post test means indicated that the growth in the subjects' critical thinking (CT) skills did not occur in the predicted direction;Possible explanations could be the small size of the study (in a statistical sense), and the short duration (one semester) of treatment;The findings do not detract from the ecological validity of the study. The study presents a strong consensus definition of CT, it shows that treatment fidelity involving an active learning component is possible, and the study points to research questions for further study. The limitations of the study can be easily rectified in future studies, and the extensive evidence in the literature suggests that these studies will yield significant results. Hopefully these results will provide the impetus for positive educational change at the micro-level in settings where educators and learners interact.