Individuals of Limulus polyphemus and Ta- chypleus tridentatus reached, respectively, the fourteen- th-instar and tenth-instar stages during our nine-year rearing experiment. Body sizes were measured using exu viae and body specimens. The results made their growth steps clear, and body sizes of the juveniles at further growth stages could be estimated. We conclude from these data that L. polyphemus males generally molt 16 times to reach maturity in the ninth year; females molt 17 times to reach maturity in the tenth year. Similarly, we conclude from the presumptive numbers of growth stages that T. tridentatus males generally molt 15 times to reach maturity in the thirteenth year; females molt 16 times to reach maturity in the fourteenth year. Although we have few data on growth stages of Tachypleus gigas and Carcinoscorpius rotundicauda, it is thought that T. gigas males molt 12 times to reach adulthood, females molt 13 times, and that C. rotundicauda reaches matu rity after the thirteenth molt in both males and females.