Urocleidus adspectus Mueller, 1936 (Ancyrocephalinae) was transmitted experimentally to Perca flavescens and development of its haptor studied at 20 °C.The oncomiracidial haptor unfolds into a ventrally directed disc with 14 marginal hooks. Hamulus rudiments appear in multinuclear oncoblasts lying immediately anterior to the larval disc. The four oncoblasts move into the disc in synchrony and rudiments develop into blades. Five days postinfection, when most larvae were found on the gills, the disc is differentiating into the haptor of adult worms. The tissue enclosing the partially developed blades of the hamuli initially projects from the posteroventral surface of the haptor; tips of the ventral blades point anteriorly or anteroventrally and tips of the dorsal blades point dorsally or anterodorsally. With further growth the tissue enclosing the blades comes to project posteriorly from the haptor; tips of the ventral blades point anteroventrally and tips of the dorsal blades point anterodorsally. During differentiation, the tips of the blades of marginal hooks II come to point ventrally. The posterolateral margins of the larval disc fold dorsally and carry marginal hooks III and IV onto the dorsal surface of the haptoral lobes. Marginal hooks V come to lie laterally on the lobes while I, VI, and VII are ventral. Seven days postinfection, the haptoral sclerites of many worms are fully developed and some worms produce eggs.