Pseudokirchneriella subcapitata is a sickle-shaped freshwater green microalga that is normally found in unicellular form. Currently, it is the best known and most frequently used species of ecotoxicological bioindicator because of its high growth rate and sensitivity to toxicants. However, despite this organism's, our knowledge of its cell biology-for example, the patterns of nuclear and cytoplasmic division in the mitotic stage-is limited. Although it has been reported that P. subcapitata proliferates by popularity forming four daughter cells (autospores) through multiple fission after two nuclear divisions, here, we report two additional reproductive patterns by which two autospores are formed by binary fission ("two-autospore type") and eight autospores are formed by multiple fission ("eight-autospore type"). Moreover, we found that cell reproductive patterns differed markedly with the culture conditions or with exposure to either of two typical toxicants, potassium dichromate (K2Cr2O7) and 3,5-dichlorophenol (3,5-DCP). The eight-autospore type occurred at the highest frequency in the early phase of culture, but it disappeared under 3,5-DCP at 2.0 mg/L. Under 0.3 mg/L K2CrO7 (Cr(VI)) the eight-autospore type took substantially longer to appear than in control culture. The two-autospore type occurred only in the late phase of culture. To our knowledge, this is the first detailed evaluation of the reproductive patterns of P. subcapitata, which changed dramatically in the presence of toxicants. These findings suggest that observation of the reproductive patterns of P. subcapitata will help to elucidate different cell reactions to toxicants.