The seasonal succession of the major phytoplankton species is described for Lake Rotongaio, a small crater lake which flows into Lake Taupo via a short channel. The dynamics of only the 4 most abundant species are described. Expressed as cell numbers,Anabaena oscillarioidesdominated in the warmer and stratified periods from November 1979 to April 1980 and again from September 1980 to the end of the observation period in January 1981. During destratification the diatomsSynedra ulnaandCyclotella menenghinianaattained maximum numbers. The green algaChlorella vulgarispeaked immediately before the September 1980 stratification. When expressed as cell volumes, however, the diatoms were always greater thanAnabaena.It is proposed that these seasonal changes can largely be explained as being long‐term versions of short‐term population fluctuations seen during thermal stratification. These are interpreted to have been responses to vertical mixing with resulting changes in the availability of light for photosynthesis in the epilimnion. During thermal stratification,Anabaenais buoyed upwards, whereas the diatoms sink to the hypolimnion. To compensate for such losses the growth rates of the diatoms must have been of an approximately similar magnitude to the growth rates leading to their winter maxima. TheSynedrapopulation decline may have been influenced by low temperature in mid‐winter. Nutrient recirculation did not appear to be an important factor for phytoplankton bio‐mass. There is little evidence for any important direct competition between the species for resources; the phytoplankton dynamics and bio‐mass appear to be mediated directly by external events and not, in any important way, by internally generated conditions.