In an effort to describe the skeletal muscle characteristics of trained triathletes, biopsies were obtained from the gastrocnemius, vastus lateralis, and posterior deltoid muscles of 11 triathletes, and 4 normally active controls. Each specimen was analyzed for muscle fiber composition, respiratory capacity (QO2), and citrate synthase activity. The mean (±SE) percentage of type I fibers for the triathletes was 59 (4.0), 63 (3.3), and 60 (2.8) in the gastrocnemius, vastus lateralis, and deltoid, respectively (P > 0.05). The mean (±SE) QO2 values in the gastrocnemius(4.4 ± 0.3 ml O2 · min-1 · g-1) and the vastus lateralis (4.1 ± 0.2) were not significantly different while the QO2 values of the deltoid (3.6 ± 0.2) were significantly lower than the gastrocnemius (P < 0.05). The mean citrate synthase activity of the deltoid (27.7 ± 1.7 μmol · min-1 · g-1) was significantly lower than both the vastus lateralis (36.0 ± 3.2) and the gastrocnemius (45.8 ± 2.1) (P < 0.05). There was a high correlation between the percentage of type I fibers and the citrate synthase activity within the vastus lateralis (r = .760) and deltoid (r = .610) (P < 0.05) but not the gastrocnemius (r = .200). No significant relationships were observed between skeletal muscle characteristics and V̇O2 max nor between skeletal muscle characteristics and performance. The results of this study demonstrate that: (1) these triathletes have a high percentage of type I fibers in all three muscle groups; (2) skeletal muscle characteristics were not highly related to laboratory or competitive performance; (3) despite a similar fiber composition, the deltoid appears to have a lower oxidative potential than the leg muscles, possibly due to the lesser use of the deltoid in normal daily activity.