The maturation and spawning of Shuttles hoppfish Periophthalmus modestus were studied from April to October, 2005 in the mud flat of Suncheon Bay, Korea. The specimens ranged in body length (BL) from 1.5 to 8.4 cm. The gonadosomatic index (GSI) of females was highest in May and decreased until September. The hepatosomatic index (HSI) and fatness index of females were high in April and then decreased to a nadir in July. The data indicate that the spawning season was from April to September. There was no significant difference in the ratio of females to males $(X^2-test,;p>0.01)$. Over 50% of the females over 5 cm in BL were sexually mature. Fecundity ranged from 1,664 to 13,428. The relationship between fecundity (F) and body length (BL) of the fish was expressed as $F=0.026BL^{2.8638};(R^2=0.79)$. The first spawning length was 4.5 cm BL.