A microsporidial infection of a transplantable plasmacytoma ascites in hamsters is reported. Intracellular parasites in tumor ascites smears, in monolayer cultures and in infected mice, rats and guinea pigs were identified as Nosema cuniculi. This is the first time that this organism has been described in tumors of hamsters. The microscopic and taxonomic characteristics for identification have been described. A method of identification by means of cell culture has been devised in view of a possible human pathogenicity for these protozoa.