An ecological study of two taxonomic varieties of Micrasterias arcuata BAILEY (Desmidiaceae, Zygnemaphyceae) - var. arcuata and var. expansa (BAILEY) NORDSTEDT -was conducted from January 1990 to March 1991 at five stations in the Acude do Jacare, an artificial pond in Sao Paulo State, southeastern Brazil. Presence of the algae in the pond was directly correlated with water temperature, thermal stratification of the water, and mixing processes in the pond. However, when the two varieties were considered separately, presence of var. arcuata was significantly directly correlated with total dissolved phosphorus, orthophosphate, chlorophyll a, and dissolved oxygen, whereas that of var. expansa was significantly directly correlated with the same environment variables above except, however, for dissolved oxygen and with inclusion of total nitrogen. Furthermore, representatives of var. arcuata were more abundant in the dry season, whereas those of var. expansa were in the rainy season; and both varieties were less abundant in rainy period 1. Both varieties occurred at the same sampling stations, but representatives of var. expansa always in higher densities. At the pond's bottom only var. expansa occurred. Occupation of different ecological niches in the pond must have greatly minimized competition between the two varieties. Comparatively smaller size and consequent greater ratio cell area/cell volume of var. expansa representatives was interpreted as a competitive advantage in relation to its nutrient requirement. Hence, their smaller sinking rates.