Abstract: The gastrocnemius muscles of rabbits on both sides were studied.Frozen transverse sections were stained with actomysin ATPase.Muscle fibers were identified as type SO,type FOG and type FG.The proportion of fiber types in each subvolume was calculated with microscopic photographs and the mean cross-sectional areas of three fiber types were measured with a image analysing system.The following were found:(1)the proportion of SO fiber type in the medial gastrocnemius was 16.7%;(2)the proportion of SO fiber type increased progressively from 11.3% of the medial subvolume of the lateral gastrocnemius to 20.8% of the lateral subvolume;(3)the proportion of FOG fiber type increased progressively from 23.2% of medial subvolume to 30.9% of lateral subvolume;(4)the proportion of type FG fiber decreased successively from 65.5% of medial subvolume to 48.3% of lateral subvolume;(5)the three fiber types were in a mosaic pattern;(6)the diameter of type SO fiber in each subvolume was the smallest (57.4-58.2 μm),the diameter of type FOG fiber larger (60.1-61.1 μm),and the diameter of type FG fiber the largest (63.6-64.5 μm).