Lake Rotongaio, a warm monomictic lake, was sampled at monthly intervals for 1 year to investigate water temperature, transparency, chemistry, zooplankton, and benthic macroinver‐tebrates. Phytoplankton composition and standing crop represented by concentration of chlorophylla,were measured for 6 months. Secchi disc values were always less than 2 m because of the large standing crops of algae dominated throughout the year byAnabaena oscillarioides.The major ion composition of the lake water indicated a source typical of a pumice aquifer. The average concentration of dissolved reactive phosphorus during the circulation period was 275 mg m‐3and the average reactive N:P ratio was 0.55. The decline in concentration of inorganic N during the breakdown of thermal stratification could not be accounted for by mixing, and 606 kg or 6.71 mg m‐3d‐lof inorganic N was lost from the lake. The zooplankton comprised rotifers (57%), crustaceans (34%), and ciliates (9%). There was 1 species of calanoid copepod, 2 species of cladocera, and 6 species of rotifer. Ninetyone percent of the benthic macroinverte‐brates were chironomids; the rest were other insects, oligochaete worms, and molluscs.