In Ireland the purple sea urchin Paracentrotus lividus (Echinoidea) typically inhabits intertidal bored holes. At Lough Hyne, Co. Cork, a population occurs associated with rock scree in the shallow subtidal zone. This study provides unequivocal quantitative evidence to demonstrate diurnal migration in a population of P. lividus at Lough Hyne, Ireland. Most size groups/age-classes did not to migrate, the youngest always and oldest never occurring under rocks. Those that did migrate between the lower and upper surface of rocks (2+, 3+ and 4+ age categories) did so in exactly the reverse circadian pattern to those described from the Mediterranean Sea.