The morphological and anatomical characteristics of the fissiparous seastar Coscinasterias acutispina were investigated at 23 sites around Japan in order to clarify the environmental factors determining asexual reproduction. The frequency of recent fission among the sites differed, although no significant differences were evident among the three major sea areas around Japan (the Sea of Japan, Seto Inland Sea, and Pacific Ocean), indicating that the spatial variability in incidence of fission might be determined on a local scale. Morphological observations revealed that the seastars endoparasitized by Dendrogaster okadai had not undergone fission recently, suggesting that fission was suppressed by parasitism. Stepwise multiple regression analysis of the frequency of seastars that had recently undergone fission revealed that nutrient conditions affected the incidence of fission. The relationships between the pyloric caeca index and arm length ratio at the individual level suggested that good nutritional conditions were required before the occurrence of fission. Additionally, regression analysis showed that high temperature was one of the triggers of fission in C. acutispina. Thus, the process of asexual reproduction in C. acutispina appears to be governed by intricate interactions among several factors, including parasitism by D. okadai, nutrient conditions, and high temperature.