Reproduction of the goby fish Periophthalmus magnuspinnatus was examined using 298 specimens collected from April to October 2005 in a mud flat of Suncheon Bay, Korea. Specimens ranged in body length(BL) from 1.3 to 9.1 cm. The gonadosomatic index(GSI) of females was highest in June and decreased until August. The hepatosomatic index(HSI) and fatness index of females were high in April and then decreased to nadirs in July. Spawning season lated from April to August and the ratio of females to males did not significantly differ($x^2$-test, p>0.05). The first spawning length was 4.5 cm BL, and the size of 50% maturity was estimated at 4.98 cm BL. Fecundity(F) ranged from 1,316 to 4,768 eggs, and the relationship between F and BL was estimated as $F=0.1562BL^{1.4068}(R^2=0.59)$.