The development of students' science reasoning abilities is a goal of science education. Researchers measure science reasoning using a variety of instruments, each with limitations and restrictions. In this study, contrasting instructional modes were analyzed for students' science reasoning development over the course of a semester. A novel instrument was developed for this. The Constructive Inquiry Science Reasoning Skills (CISRS) test improves on other reasoning instruments in that it is (a) independent of mathematical or manipulative skills, (b) relevant without being dependent on prior content, (c) easily administered to a large group of students in a short period of time, (d) developmentally appropriate to the user, and (e) discerning of reasoning abilities. Students enrolled in contrasting styles of introductory biology courses were tested using the CISRS test. By this measure, students in the inquiry-based course made encouraging gains in reasoning ability similar to that of their peers in the major course. (Contains 4 tables.)