Activity rhythms of two cirolanid isopods, Excirolana armata and Excirolana braziliensis, were studiedbased on both seasonal field observations and laboratory experiments, at an exposed microtidal sandy beach inUruguay. The natural emergence patterns of both species were observed in the field for 1 year, twice in each season,and correlated to sea level, expected tidal cycles and diel cycles. Laboratory experiments were carried out in order todetect endogenous rhythms of activity and observe how emergence of both species was affected by changes in lightand/or sediment thixotropy. We also compared behavioral strategies of sympatric species that occupy different beachlevels. Sea level (and thus swash zone position) during field sampling did not follow expected tidal cycles for mostsampling occasions. E. armata was observed in activity most of the time, but activity only correlated with sea level onthree out of eight occasions, and only once was correlated to expected tidal cycle. Laboratory results showed thatemergence under constant conditions was rare; changes in sediment thixotropy stimulated emergence, but theresponse was not cyclical; light had little effect on this response. On the other hand, E. braziliensis was fairly scarce inthe water column, but swimming individuals were observed always during the night. They displayed an endogenouscircadian activity pattern in the laboratory which augmented in response to changes in sediment thixotropy. The naturallight/dark cycle modulated both spontaneous and response emergence by increasing day/night differences in activity.In this study E. armata, a midlittoral species more exposed to sea level variations, seemed to rely entirely on differentphysical and/or biological cues to trigger emergence at the appropriate time. E. braziliensis, found mostly in the upperintertidal zone, emerged in a circadian rhythm, which was stimulated by changes in sediment thixotropy and reinforcedby light cycles. The results of this study led us to conclude that on microtidal, unpredictable beaches, local physicaland biological factors can combine to determine different activity strategies in organisms from different intertidal levels.