A Japanese water strider, Aquarius paludum, exhibits the wing dimorphism, long- and short-winged morph, coexisting on the open water in summer season. In order to clarify the difference in reproductive strategy between morphs, females of each morph was reared in the laboratory. Immediately after eclosion, females of both morphs loaded a few immature eggs in their ovaries. The number of immature eggs was increased with the age, and 50 eggs were loaded after 5 day old females of each morph. Sub-mature eggs were found in the ovaries of 3 day old long-winged females, while 1 day old short-winged females. Mature eggs were observed in 5 day old and 1 day old for long- and short-winged females, respectively. Although the number of mature eggs was increased with age, long-winged females loaded lower number of mature eggs than short-winged females. When a female was not sexually mature, they showed mate refusal behaviour to the male approaching. Females of both morphs after 11 day old accepted to copulate. No significantly difference in the copula duration was observed between morphs. They started to oviposit 2 days after the mating. The size of the first clutch was depended on the number of mature eggs loaded. Their first clutch size in the long-winged females (18 eggs) was significantly lower than that in the short-winged females (41 eggs). However, after the fourth clutch, there were no significantly differences in clutch size between the morphs. The small clutch size in the first clutch and late egg maturation of the long-winged females might be related to flight performance for migration, while large clutch size and fast egg maturation of short-winged females might be evolved to increase reproductive success.