Wallowing is a common behavior of American bison (Bos bison). Past explanations and current hypotheses suggested for wallowing behavior include grooming behavior associated with shedding, male-male interaction (typically rutting behavior), social behavior for group cohesion, play behavior, relief from skin irritation due to biting insects, reduction of ectoparasite (tick and lice) load and thermoregulation. We monitored circannual and circadian patterns of wallowing frequency by American bison during 1996–1997 in the tallgrass prairie region of eastern Kansas. Wallowing activity increased from April to late June or July (during 1996 and 1997, respectively), decreased during midsummer, peaked again in September, decreased from September to October and then remained low from November to March. Diurnally, wallowing was low in early morning, increased to a peak in early afternoon and then decreased during mid afternoon and evening. Within the herd adult males wallowed more frequently than adult females and both adult males and females wallowed more frequently than yearlings. We observed behaviors that were consistent with all of the hypotheses previously suggested to explain wallowing behavior by bison. Based on our observations we suggest that the alternate explanations for wallowing behavior are not mutually exclusive. However, only the relief from biting insects hypotheses was consistent with both the circannual and circadian patterns of frequency of wallowing by American bison.