Although movement of individuals has important consequences on population dynamics and various ecological interactions, it is often difficult to quantify fully. We investigated the temporal variation in the number of the amphipod Corophium volutator swimming in the water column during periods of immersion over an intertidal mudflat in the Bay of Fundy, Canada, in spring-summer 2006. Swimming is an important mode of dispersal, since the number of swimming amphipods can peak at over 30,000 individuals within a 20-cm-diameter, stationary plankton net over a period of immersion of ∼4 h. Amphipods swim throughout spring-summer, but abundance in the water column is less in May than in the other months. As well, amphipods swim during the day and night, but the number swimming shows periodicity in relation to diel time of high tide, with peaks when high tides occur around 1:45 am. Finally, the number of amphipods swimming shows periodicity in relation to lunar cycles, with peaks around the time of new moon and full moon. We developed a statistical model describing the swimming activity of C. volutator based on month, diel time of high tide, and day of the lunar calendar. The model accurately predicts the timing of peaks, but does not predict well the amplitude of the highest peaks. Overall, the model gives a very good approximation of the number of swimmers (61% of the variation is explained) and provides a strong basis for future modeling of spatial population dynamics of C. volutator.