Two spore stages in the life cycle of Myxobilatus gasterostei, a ubiquitous myxosporean parasite of three-spined sticklebacks, were identified by matching small subunit ribosomal RNA gene sequences of actinospores from a worm with myxospores from fish. A Nais communis oligochaete collected in the Willamette River, Oregon, USA was found to produce a triactinomyxon-type actinospore which was distinguishable from previous records by its large size (approximately 500 microm across) and number of germ cells (approximately 500). Its small subunit ribosomal RNA gene sequence was >99% similar to M. gasterostei from Europe. Two of ten sticklebacks from the Willamette had renal infections with Myxobilatus myxospores which were smaller than the type description of M. gasterostei Parisi (1912) but were consistent with subsequent records. Primer Mg1097r was designed to selectively amplify M. gasterostei in the presence of another common kidney myxosporean, Sphaerospora elegans. DNA sequences of spores from the fish were identical to each other and were 99.8% similar over 2,112 nt to the spores from the oligochaete. The 0.2% sequence divergence comprised polymorphisms at five loci, which suggested that multiple alleles were present in the parasite population. This is the first Myxobilatus species shown to have two spore stages in its life cycle and to infect an invertebrate. The infected oligochaete underwent paratomic fission to produce two daughter worms with parasite stages in their intestinal epithelia, which suggested that M. gasterostei may be sustained and dispersed within the invertebrate host population.