The effects of preventing female helmet crabs, Telmessus cheiragonus from mating during one breeding season on egg production and egg viability were assessed by investigating the egg clutches of the females isolated from males. A total of 38 females and 18 males were collected in April before the mating peak in 1996 and 1997 in the sublittoral zones of Usujiri, southern Hokkaido, Japan. Eighteen females were held together with 18 males (mating group) and 20 females were isolated from males (non-mating group) during about 7 months of captivity. Subsequently, 11 females of the mating group molted, copulated and produced an egg clutch, and 10 females of the non-mating group molted and spawned without mating. Egg clutch weight and egg diameter did not differ significantly between these two groups. There was also no significant difference in the percentage of viable eggs in an egg clutch between the groups. These results show that preventing female T. cheiragonous from mating during one breeding season has no effect on either clutch size or % viable eggs in a clutch. Sperm storage over one mating season in this species may be advantageous because it allows multiparous females, especially larger ones which could molt and copulate less frequently, to ensure their eggs will be fertilized.