Cultural and caryological investigations on Sphacelaria furcigera from Hoek van Holland give evidence of a slightly heteromorphic diplohaplontic life-history in this species. A relatively slender (13.5—31 μ) haploid gametophytic phase alternates with a more robust (19—41 μ) diploid sporophytic phase. Female gametophytes form plurilocular macrogametangia at 12˚ C and 4˚ C, male gametophytes form plurilocular microgametangia at 12˚ C and 4˚ C. Zygotes of female macrogametes and male microgametes grow into diploid sporophytes which form unilocular meiotosporangia at 4˚ C. Meiosis takes place in the initials of these sporangia. About 50 % of the spores produced by them grow into male gametophytes, about 50 % into female gametophytes. Propagules are formed at higher temperatures, i.e. at 12˚ C and 20° C. In nature gametangia are formed during the winter half of the year, and propagules during the summer half of the year. Female gametes can develop parthenogenetically into new female gametophytes, but also into haploid plants forming unilocular zoidangia instead of plurilocular macrozoidangia. Such plants are repeatedly formed and represent dead ends in the life-history of S. furcigera. The result of our investigations are summarized in fig. 4. Sphacelaria britannica Sauv. and S. saxatilis (Kuck.) Kuck. ex Sauv. are synonyms of S. furcigera Kütz.