To address whether a fish host is involved in the life cycles of malacosporeans of the genus Buddenbrockia, cohabitation experiments with different bryozoan and fish species were conducted. Samples were analysed by malacosporean-specific PCR, partial sequencing of the 18S rDNA, and light and electron microscopy. Co-habitation challenges with bryozoans resulted in malacosporean infections detected mainly in the kidney of common carp (Cyprinus carpio) and minnow (Phoxinus phoxinus). Sequences of the minnow parasite and of worm-like Buddenbrockia stages in Plumatella repens were identical and showed 99.5% similarity to Buddenbrockia plumatellae and 96.3% similarity to the sequence obtained from carp. One sample, comprising 4-5 zooids of statoblast-raised bryozoans cohabitated with infected carp was PCR-positive, but no overt infection could be observed in the remaining colony. Light and electron-microscopy of kidney samples of infected minnows revealed single cells within kidney tubules, whereas in carp, sporogonic stages were found in kidney tubules. Phylogenetic analysis of the Buddenbrockia spp. known to date placed the carp-infecting species at the base of the B. plumatellae clade, but low posterior probability makes this node questionable. The present study showed that Buddenbrockia spp. were able to infect cyprinid fish, showing stages in kidney-tubules strikingly similar to those of T. bryosalmonae.