Previous findings of time-of-day differences in athletic performance could be confounded by diurnal fluctuations in environmental and behavioral "masking" factors (e.g., sleep, ambient temperature, energy intake). The purpose of this study was to examine whether a circadian rhythm in swim performance existed that was independent of these masking factors. Experienced swimmers (n=25) were assessed for 50-55 consecutive h in the laboratory. The swimmers followed a 3-h "ultra-short" sleep/wake cycle, involving 1 h of sleep in darkness and 2 h of wakefulness in dim light, repeated throughout the length of observation. The protocol distributes behavioral and environmental masking factors equally across the 24-h period. Each swimmer was scheduled to perform 6 maximal-effort 200-m swim trials that were distributed equally across 8 times of day (n=147 trials). Each trial was separated by 9 h. A cosine fit of intra-aural temperature data established the body temperature minimum (T_min). Swim performances were z-transformed and compared across the 8 times of day and across twelve 2-h intervals relative to T_min. ANCOVA, controlling for trial number, revealed a significant (p<0.001) pattern in swim performance relative to both environmental and circadian times of day. Performance peaked between 5 and 7 h before the T_min (approximately 2300 h) and was worst between 1 h before to 1 h after the T_min (approximately 0500 h). Mean swim performance was 169.5 sec; circadian variation from peak to worst performance was 5.8 sec. These data suggest a circadian rhythm in athletic performance exists independent of environmental and behavioral masking effects.