The goal of this study was to determine: (1) the extent to which channeled whelks (Busycotypus canaliculatus) expressed biological rhythms in the field, based on catch data; and (2) the degree to which their daily activity rhythms in the laboratory are influenced by an internal biological clock versus exogenous environmental cues. In the field studies, a timelapse video system was used to quantify when whelks entered traps during deployments in the coastal waters near Cape Cod, MA (n = 23 trials). Most of the whelks observed entering traps did so during the night, shortly after sunset on the first day of deployment. There was also a tendency for whelks to enter traps more during high tides. In the first laboratory experiment, when whelks were held in flow-through seawater tanks receiving ambient seawater with prevailing environmental cues, 73% (8/11) expressed a tidal rhythm in both a light:dark cycle (13L:11D) and constant darkness (DD) conditions. In the second laboratory experiment, when whelks were tested in tanks held at a constant temperature and 11L:13D, 45% (9/20) were nocturnally active. When these same whelks were subsequently exposed to DD conditions, only 30% (6/20) expressed a circadian rhythm. In the third laboratory experiment, when whelks were held in 19-L (5-gallon) buckets, placed inside a larger holding tank, in an environmentally controlled cold room under 14L:10D illumination, 60% (6/10) of the whelks were nocturnal, but a circadian rhythm only persisted in 30% (3/10) of them in DD conditions. These data, taken together, indicate that, whereas this species likely possesses a circadian clock that plays a role in controlling their patterns of locomotion, prevailing environmental cues also have a large impact on their activity and catchability.