Studied size variation in the strıped mouse Rhabdomys pumilio, a diurnal herbivorous murid, across southern Africa using the greatest length of the skull (GTL) as a measure of body size. There was a positive correlation between GTL and the mean minimum temperature of the coldest month (July), contrary to Bergmann's rule, but there was no significant correlation between GTL and either mean maximal annual temperature, mean maximal temperature of the hottest month (January), altitude or annual rainfall. There were differences in size between samples of different biotic regions: Animals from the south west Cape were largest, followed by those from the Namib desert, forest, south west arıd zone ,and the savanna ,respectively. Animals from the zone of sympatry with Lemniscomys griselda, a larger herbivorous diurnal murid, were significantly smaller than those from allopatric zones. It is suggested that character release is a primary factor in determining body size of R. pumilio in southern Africa.