Body fat lessens underwater body weight and may offer an advantage for swimming performance. The present study was undertaken to measure separately V0_2 for buoyancy and that for propulsion during swimming in the swimming flume and to elucidate the advantage of lower underwater body weight in female. Three male swimmers and three female swimmers participated as the subjects. VO_2 was measured during free style swimming at a constant speed of 0.6,0.8 and l.0m/sec.Underwater weight was increased stepwisely by loading an extra-weight around the subject's waist or decreased by suspending a weight which pulls the waist upward via a wire and pulleies. VO_2 at a given speed depended proportionally on the underwater weight. VO_2 for propu1sion was estimated by subtracting resting V0_2 from the intercept on the ordinate, and V0_2 for buoyancy was calculated from the slope.1) VO_2 for buoyancy was independent of swimming speed and the average value for female swimmers was much smaller than that for male swimmers (352±140m1/min for male, 186±83m1/min for female). This difference in VO_2 for buoyancy depended largely on the difference in underwater weight as the calculated values of VO_2 for buoyancy per kg of underwater weight revealed much smaller difference between sexes (117±46m1/min for male, 91±36m1/min for female).2) VO_2 for propulsion increased exponentially with increasing speed. The increasing rate was larger in female than in male. This is probably because of relative inferiority of swimming ability in the female group in this study.3) The rate of propulsion VO_2 to total VO_2 during swimming was larger in female than in male. This represents the advantage of lower underwater weight in female for swimming. This result offers the probable explanation for the discrepancy which exists in male-female ratio of the world records between swimming and running.