We hypothesized that apple snails would change life-history traits in the presence of common carp to reduce or avoid predation risk. Carp of about 150 mm body length were released in small plots set in a rice field in southern Japan at 0, 0.2, or 0.6 carp/m2. Egg-mass size, proportion of hatched eggs, and duration of hatching of the snail were measured once or twice a week from July to September. Snails collected in traps were used to estimate both snail density by the Jolly-Seber mark-recapture method and distribution of shell lengths in each plot. The weight of the snail was regressed on a size-weight equation, and the snail biomass was determined by multiplying the estimated density and the regressed weight. The reproductive effort of the snails was calculated as the number of eggs divided by the biomass of adult females. The egg mass size and reproductive effort were significantly increased in the presence of carp. These increases were considered as life-history changes of apple snails in the presence of a predator.