Proliferative kidney disease (PKD), caused by the malacosporean parasite Tetracapsuloides bryosalmonae, causes significant losses among salmonids in Western Europe and North America. The role of salmonid fish in the life-cycle of this parasite has been conjectured upon for over a quarter of a century. To examine whether fish can transmit the infection to bryozoans, the known invertebrate host, water containing parasitized brown trout Salmo trutta was pumped into tanks containing colonies of Fredericella sultana collected from the wild. The specific parasite-free status of these colonies being first assessed, by PCR and prolonged laboratory culture. After 6 weeks exposure to the brown trout aquarium effluent, portions of these colonies displayed overt infections with T. bryosalmonae. This was in contrast to control bryozoans, derived from the experimental colonies prior to exposure, which remained T. bryosalmonae negative. In addition, spores obtained from the experimentally infected colonies were exposed to naïve rainbow trout, resulting in clinical PKD, thus completing a cycle of transmission. During the experiments, the infection was noted to inhibit statoblast formation within bryozoans and appeared to be pathogenic, finally killing the bryozoan host. These findings indicate that fish can transmit the parasite to bryozoans and are an integral part of this parasite's life-cycle.