Patterns of in situ fertilization success with respect to high and low tide periods are reported for the intertidal red seaweed Gracilaria gracilis which inhabits rock pools. Cystocarp (zygote) production was experimentally compared at high tide and low tide in both high- and low-shore tide pools at two sites using ten virgin female plants. High- and low-shore pools showed different patterns with respect to tide period. High-shore pools showed significantly greater numbers of cystocarps at low tide than at high tide whereas cystocarp production did not differ between high and low tide periods in low shore pools. The possible mechanisms of gamete release are discussed in light of these results.