Muscle biopsies were obtained from the vastus lateralis muscle of 5 female collegiate field hockey players and 5 untrained female students. The fibers were classified histochemically as fast-twitch-oxidative-glycolytic (FOG), fast-twitch-glycolytic (FG) and slow-twitch-oxidative (SO). The fibers were found to be similar to those of males in distribution and histochemical properties, but were smaller. In the women athletes all 3 fiber types were larger than the respective fibers in the controls. Also, the athletes had a much higher percentage of oxidative fibers (SO + FOG), 83% vs. 46%. A direct relationship between fiber size and oxidative activity was observed in fast-twitch fibers, whereas the reverse was found in slow-twitch fibers.