Sorus abscission and associated spore release were monitored in the laboratory from Nereocystis luetkeana (Mertens) Postels et Ruprecht plants collected at three sites in central California, U.S.A. Nearly 80% of the sori were released in the interval between 2 h before sunrise and 4 h after sunrise. This dawn abscission pattern was observed on each day and from plants collected at all sites. Short-term observations in nature confirmed the pattern. Sori from individual plants were usually abscised at intervals of 2-4 days. Sori often began releasing spores before abscission and, in nature, probably continue to release spores as they are falling through the water column. > 50% of the spores released from abscised sori in the laboratory were released in the 1st h after abscission at a mean rate two to four orders of magnitude higher than previously reported for related macroalgae; nearly 95% had been released within 4 h. This sorus abscission-spore release mechanism is probably adaptive in maximizing the photosynthetic potential of the spores, maximizing the potential dispersal of some of the spores, and ensuring that a proportion of the spores will be retained near the parents. © 1989.