A general ecological problem is considered: how long can a photoautotrophic microalga, incapable of producing a resting spore (stage), retain its viability in the dark following removal from the euphotie zone ? Nine coastal diatoms, including some capable of producing resting spores, were kept in the dark for 90 days at 15~ and their growth (viability) checked at periodic intervals upon reillumination. Seven of the 9 diatoms retained their viability for 90 days; generation time of illuminated cells then ranged from 2.5 to 10 days. Skeletonema costatum survived only 7 weeks of darkness. Based on the present and published observations, dark survival of this species is inversely related to temperature; it survives at least 24 weeks at 2 ~ and from 1 to 4 weeks at 20 ~ None of the species was observed to grow in the dark. The effects of temperature and light on dark survival, and of darkness on the chemical composition and photosynthesis following reillumination as reported in the scattered literature are evaluated. Together with the present observations, it is suggested that dark survival of photoautotrophic mieroalgae: (1) varies between species; (2) may be temperature dependent in some species,. . as. in S. costatum; (3). may. be. prolonged by periodic. fllumlnatmn at subcompensatlon intensities for photosynthesis, as shown in Dunaliella tcrtiolecta. The potential ecological significance of these findings is also considered, should these in vitro results apply to natural populations.