At its northernmost distributional limit in northeastern America, in Newfoundland, Bryopsis hypnoides Lamour. appears to have a heteromorphic, monophasic life-history. Zygotes resulting from fusion of anisogametes develop in culture into prostrate filamentous germlings, these also reproducing sexually by anisogametes to form a new generation of the prostrate type. Erect Bryopsis later grew from the prostrate phase: asexual stephanokontic zooids were entirely absent. The life-history in Newfoundland compares in part with that for B. hypnoides from northern parts of its European range ; the presence of sexual reproduction in the prostrate phase in Newfoundland, however, is unique in Bryopsis and is not apparently a temperature dependent phenomenon. In the field Newfoundland B. hypnoides reaches sexual maturity in winter at temperatures less than 0°C. Germling phases studied in culture could not be located in the field. The question is raised as to the possible genetic isolation of Newfoundland B. hypnoides from populations in Europe.