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Two species of the genus Acomys, cahirinus and russatus, co-occur in En Gedi. A. cahirinus is nocturnal, whereas A. russatus is diumal. This pattern of temporal partitioning has been ascribed to competitive exclusion of A. russatus by A. cahirinus. Study of the ecological basis for this phenomenon and its mechanisms, however, is still required. Here we report preliminary results of longterm research that we undertook in order to establish precise activity patterns, body temperature rhythms, spatial distribution, food habits, and population dynamics of the two species. Population densities of both species are quite row, and much lower than those found in agricultural areas. A. russatus was; found mainly in a creek bed strewn with boulders, and also on the hillside among boulders. A. cahirinus appeared to be more evenly distributed, and was found also among smaller rocks; determination of precise spatial distribution patterns, however, requires more thorough study. While A. cahirinus was active almost only during the night, A, russatus was active during the entire day during the winter months. Both species exhibit body temperature rhythms that conform to those expected of a nocturnal rodent; however, A. russatus body temperatures rise earlier (several hours before sunset) and drop later than those of A. cahirinus, and only reach their minimal level several hours after sunrise. This difference may reflect an adaptation to a diurnal activity pattern.