We have recently demonstrated that people with a high percentage of Type I muscle fibers display a relatively high muscular efficiency when cycling. These individuals generate a relatively high muscular power output at a given steady-state level of oxygen consumption and caloric expenditure. The purpose of this study was to directly determine the extent to which differences in muscle fiber composition and efficiency influence endurance performance in competitive cyclists. The percentage of Type I and II muscle fibers was determined from several biopsies from the vastus lateralis which were histochemically stained for ATPase activity. During a laboratory performance test, 14 endurance trained cyclists (mean±SE; V̇O2max, 5.2±0.11/min; body weight, 74±1 kg) cycled an ergometer for 1 h at the highest work rate they could tolerate. V̇O2 and RER were simultaneously measured using open circuit spirometry for calculating caloric expenditure. Subjects were divided into two groups of seven according to their muscle fiber type composition: High% Type I Group (>56% Type I fibers); Normal% Type I Group (38-55% Type I fibers). Each subject from High% Type I Group was paired with a subject from the Normal% Type I Group according to their similarity in V̇O2max, blood lactate threshold and average V̇O2 maintained during the 1 h performance test. Both groups averaged 4.5±0.11/min during the 1 h performance test (i.e., 86-88% V̇O2max). However, the High% Type I Group, which possessed an average of 72±3% Type I fibers, was able to maintain a 9% higher power output (i.e., 342±9 vs 315±11 watts; p < 0.001) than the Normal% Type I Group which possessed an average of 48±2% Type I fibers. Gross efficiency was thus significantly higher in the High% Type I Group compared to the Normal% Type I Group (i.e., 21.9±0.3% vs. 20.4±0.3%; p < 0.001). We conclude that a high percentage of Type I muscle fibers improves endurance performance ability by significantly increasing the power output generated for a given rate of oxygen consumption and energy expenditure.