Nephromyces Giard is a diverse collection of funguslike microbial cells, endosymbiotic in the ductless renal sac of molgulid ascidian tunicates (phylum Chordata). Though Giard considered Nephromyces a Chytridiomycete, the taxonomic affinities of these peculiar cells remain uncertain. In Molgula manhattensis DeKay, at least seven Nephromyces cell types can be distinguished. Despite their diverse appearance, these cell types are not separate taxa, but instead are separate life-history stages of a single organism. Developmental roles for these life-history stages are discussed, as are the possible taxonomic relations of Nephromyces. A new Nephromyces cell, a motile but nonflagellated infective stage, is described. The experiments reported here confirm earlier suggestions that the molgulid renal sac is not merely the site of an occasional or random microbial infection. Instead, this organ is the locus of an exclusive, presumably coevolved, association between two taxa: Nephromyces and the tunicate family Molgulidae.