Background: Few studies have examined physical activity during the transition from high school to college. Purpose: To examine changes in physical activity and physical activity patterns among females during the transition from high school to college. Methods: Sixty-nine females (age 18.2 [plus or minus] 0.4 years; body mass index 21.8 [plus or minus] 2.6 kg/m[superscript 2]; 84% Caucasian) at a large university in the south central United States participated in this prospective longitudinal study. They completed a questionnaire at the beginning of their freshman and sophomore years of college, recalling their participation in physical activity during the previous 12 months. Results: Weekly time spent in moderate, vigorous, and moderate-to-vigorous physical activity declined between high school and college (p less than 0.01). Physical activity participation also differed by semester (p less than 0.01), with a significant decline during the summer between high school and college (p less than 0.01), and an additional decrease during the first semester of college (p less than 0.01). Discussion: Participants' physical activity significantly decreased from high school to college, with the largest decline occurring during the summer between high school and college. Translation to Health Education Practice: The transition from high school to college is a critical time to promote physical activity among women. (Contains 1 table and 1 figure.)