SYNOPSISIn a survey of microsporidian parasites of trematode larvae of Malaysian fresh‐water snails, species of Nose‐matidae were found 7 times in rediae from 425Lymnaea rubiginosa, 6 times in rediae from 97Indoplanorbis exustusand not at all in other snails. There were 3 species of Microsporida in all: one, found inEchinoparyphium dunni, Echinostoma hystricosumandEchinostoma malayanumwas identified asNosema eurytremaeCanning; another, which was found inE. dunniandEchinostoma audyi, had paired nuclei (diplokaryon form) in sporogony only and was named a new species,Nosema vasicolasp.n.; the 3rd, fromE. hystricosum, lacked diplokaryon nuclei throughout development, was considered to belong to a new genus and was namedUnikaryon piriformisgen.n., sp.n.Nosema eurytremaewas transmitted experimentally toFasciola hepaticarediae inLymnaea truncat