The mudskipper, Periophthalmus modestus, inhabits the intertidal zone and is active on the surface of mud flat during low tides. We investigated relationships between the behavior and environmental variables during low tides from daily and annual aspects. The observation was done in two distant sites where tidal cycles and population densities are different. The emergence of the mudskipper on the surface of the tidelands was influenced by ambient temperature; low temperature inhibited emergence and locomotor activity of the mudskipper. The mudskipper retreated into the mud as the time went on after emersion of the tideland in order to avoid increase of body temperature and desiccation of the epidermis. Therefore, the emergence of the mudskipper seems to be directly suppressed by drying up of the mud surface. There were no differences in the diel and annual changes in the activity pattern between the populations. The emergence exhibited synodic changes in both areas, and it was correlated not with lunar phase but with tidal amplitude, peaking at the time of lower ebb tides with high air temperature.