We investigated the effects of environmental conditions, cultivation history and habitat area on the spatial distributions of alien plants in Chagusaba (semi-natural grasslands which are maintained for hay harvesting) in west-central Shizuoka, Japan. The number of alien plant species and probabilities of occurrence of Solidago altissima L., Festuca myuros L., Lolium multiflorum Lam. and Briza minor L. were much higher in formerly cultivated Chagusaba than in Chagusaba that had never been cultivated. Calcium content in soil was positively correlated with the number of alien plant species, and with the probabilities of occurrence of S. altissima and L. multiflorum. Probabilities of occurrence of S. altissima, F. myuros, L. multiflorum and B. minor increased with increasing openness. Much effort should be allocated to control alien plants on calcium-rich and open, formerly cultivated sites.