Based on detailed quantitative distributional data for the North Atlantic and qualitative data for the North Pacific, a hypothesis of winter temperature control of reproduction and distribution is formulated for the crustose coralline alga Clathromorphum circumscriptum. The hypothesis, that winter temperatures below 2–3 C are required for reproduction, was tested by growing plants for extended periods under various light and temperature conditions. Regimes from constant light cycle (12–12) and intensity (1200 lux) at a temperature of 12–13 C, to reduction of both light and temperature over a 5 month interval to 6–18 hr light-dark periods, 130 lux and 0 C, were employed in a manner approximately simulating the natural pattern in the shallow sublittoral of the Maine coast. Although conceptacle production was initiated in those tanks where the light cycle was reduced from 12 to 6 hr per day, mature conceptacles with bispores developed only in the tanks where the temperature was reduced. The most prolific plants were those subject to reduction of light cycle and intensity. In tanks having neither temperature nor light cycle reduction, only a very small percentage of primordial conceptacles were initiated and none of these developed to maturity. The productivity of Clathromorphum circumscriptum, based on oxygen exchange measured with a precision polarographic electrode, was examined at light intensities ranging from 0–4000 lux and temperatures ranging from 0–20 C. The compensation point was reduced markedly with temperature, reaching levels of only 35 lux near 0 C. This indicates a high level of efficiency in net productivity at very low temperatures, and suggests that under arctic conditions holoautotrophic plants probably can survive even where ice cover and winter darkness significantly reduce total light received.