The effects of low temperature on morphogenesis and cellular differentiation of Dictyostelium discoideum were examined. During incubation at 5 degrees C, the vegetative and preaggregation cells never developed, but cell masses at the aggregation or slug stage developed to form hemispherical, or dumbbell-shaped multicellular structures. By staining with FITC-antispore IgG, the structures formed after 10 days of incubation of tipped aggregates at 5 degrees C were found to be composed of 90% spores, 5% prespore cells and 5% non-stained cells. Since only 20% of the total cells constituting the tipped aggregate had been prespore cells at the beginning of incubation, this showed that spore differentiation proceeded even at low temperature, while stalk differentiation was completely inhibited. Similar results were obtained when the cells were incubated at 3 degrees C. However, at 0 degree C, morphogenesis and cellular differentiation did not occur, although most of the prespore cells at the late culmination stage differentiated incompletely into spores. Possible reasons for the high proportion of spores being induced by low temperature are discussed.