Latitudinal clines in sexual dimorphism have been reported in some taxa. Although it has been hypothesized that these reflect latitudinal clines in the strength of sexual selection pressure, there is no research on latitudinal variation in mating behaviors that supports this view. In the medaka Oryzias latipes, lower-latitude populations are more sexually dimorphic than higher-latitude populations, suggesting stronger sexual selection pressure at lower latitudes. In this study, male mating competitiveness and females mate choosiness were compared among three latitudinal populations (Aomori, Fukui, and Okinawa) of this fish. Laboratory mating experiments revealed that males from lower-latitude populations show more frequent fighting and courtship behaviors than males from higher-latitude populations. On the other hand, females from lower-latitude populations tended to spend a longer time on accepting a male than females from higher-latitudes, especially when mating with higher-latitude males. Furthermore, lower-latitude females refused male courtship more frequently, especially from higher-latitude males. These results suggest that lower-latitude males and females are more competitive and choosy, respectively, supporting the view that sexual selection pressure is stronger at lower latitudes. Reasons for the latitudinal cline in sexual selection pressure will be discussed.