Changes in body shape of the rotifer Keratella quadrata in a pond were recorded over the course of half a year (end of November 1981 to beginning of June 1982). Body and caudal spine lengths increased from November until the end of February, and then decreased until June. Such changes were not significantly correlated consistently positively or negatively to water temperature, pH, hardness, food levels or copepod numbers. The individuals present during the first half of the study period corresponded most closely to the literature descriptions of the group or subspecies quadrata, while those present during the second half resembled the form, group or variety dispersa. The first half of the study period was characterized by water of relatively low temperature, low pH and high hardness, while, for the second half, temperature and pH increased and hardness decreased.