En masse foraging migrations of Ligia occidentalis Dana were conspicuous events on rocky beaches near Puerto Peñasco, Sonora, Mexico. Experiments were conducted to determine which factors of the environment contained the regulating rhythm for the migration, if indeed the event were cyclic. Experiments were conducted in the laboratory using a tide-simulation aquarium and in the field using mark and release techniques. A time-lapse photography system was used in the field to quantify isopod activity for comparison with environmental factors. A regular cycle for the foraging migration was found. It was in tune with both insolation and tidal cycles so that the isopods began their migrations at the low tides which commenced uncovering the lower intertidal zones during daylight. This resulted in a sudden shift of isopod activity from the evening low tide to the morning low tide when the tides reached spring phase.