Patterns of natural gamete release and fertilization success are reported for the open-coast macrophyte Fucus distichus L. (formerly Fucus distichus ssp. distichus), which is restricted to tide pools in the upper intertidal zone. The release and settlement of eggs and zygotes of E distichus occurred during daytime low tide periods (DLT), largely on days when the low tide fell between 10:OO and 14:OO h EST; very few gametes were released when high tides fell during the same time interval. Gamete release during nighttime low tides was very low. During the DLT series, gametes were released in all pools and this was better correlated with the time of day than with time of low tide per se.The timing of fertilization showed considerable variation both within and between pools during a single DLT. No relationship between either pool temperature or osmolality and gamete release was evident, and only small episodes of gamete release coincided with the times of full or new moons. The restriction of gamete release to periods of very low water motion, when tide pools are ~solated by the low tide, prevents gamete dilution by water exchange and turbulent flows, and results in external fertilization success between 78 and loo'%) with low levels of polyspermy ( l to 5%). Dispersal of zygotes among tide pools may be facilitated by the low winter temperatures during reproduction, which retard adhesive production and zygote attachment. Our results demonstrate that intertidal organisms, living in habitats characterized by periodic, turbulent flow regimes, Inay achieve high levels of fertilization success by releasing gametes under optimal hydrodynamic conditions for sperm-egg encounters.