Zygotes and 1- to 3-day-old embryos of Fucus edentatus were frozen in aqueous medium to −2°C (freezing point), −5°C, −10°C, −15°C, and at −15°C for 2 hr. Embryos aged 1–6 days were also exposed to −25°C for periods from 6 hr to 14 days. No appreciable mortality of zygotes occurred at the two highest temperatures, but survival of exposure to −10°C varied from 34 to 92%. Survival declined further at −15°C and was usually nil after 2 hr at this temperature. A similar pattern was observed in occasional trials with F. distichus subsp. distichus, F. serratus, and F. vesiculosus. Winter zygotes withstood severe cold better than summer zygotes, although tolerance varied considerably within a season. Cold-hardiness of zygotes increased after storage at 0°C for periods from 1 hr to 3–5 days prior to freezing, during which time the zygotes remained ungerminated. Pronounced changes in cold-hardiness between fertilization and germination were observed in zygotes of F. serratus and F. vesiculosus. Cold-hardiness increased markedly on germination and survival at −25°C was usually nearly 100% by age 3 days, although localized injury and general retardation of growth frequently occurred. Repeated freezing of 5-day-old embryos had no cumulative adverse effects, and 6-day-old embryos tolerated 7 days at −25°C with 55% survival.