Two highly trained groups, competitive swimmers (N = 6) and triathletes (N = 5), were compared to evaluate the significance of the propelling efficiency as a performance determining factor in swimming. Using regression equations, the groups were compared at equal power input (1000 W). The groups did not differ in gross efficiency, stroke frequency, and work per stroke. There was a difference in distance per stroke (1.23 m vs 0.92 m) and mean swimming velocity (1.17 m.s-1) vs 0.95 m.s-1). The difference in swimming speed between the two groups can be explained by the fact that the competitive swimmers used a much higher proportion of their power output to overcome drag (49 W vs 35 W). At the same time, the competitive swimmers expended less power in moving water backwards (32 W vs 45 W). This difference in apportionment of the power output was characterized as the propelling efficiency (power used to overcome drag/total power output). Mean (+/- SD) propelling efficiency for the competitive swimmers was 61 +/- 6% but was only 44 +/- 3% for the triathletes. The results suggest that on average the better swimmer distinguishes himself from the poorer one by a greater distance per stroke rather than a higher stroke frequency. It is concluded that triathletes should focus their attention on their swimming technique rather than their ability to do work. The distance per stroke might be a simple criterion to evaluate the improvement in skill.