The hypothesis of a critical day in biorhythm theory predicts that skill performance would be lower than normal on a physically critical day. Participants in an archery league were observed in 346 events. Indoor archery performance provided scores identical to the goal of the task and unaffected by environmental conditions or other competitors. Above-average performances on 34 physically critical days did not differ significantly from the population proportion of above-average performances. No support for the hypothesis was provided by this observation of archery performance.