Polychromophilus murinus, a malaria parasite of Chiroptera is reported from Myotis daubentoni in England. The vector was suspected to be the ectoparasitic Nycteribiid fly, Nycteribia kolenatii. N. kolenatii collected from wild-caught M. daubentoni were found to have oocysts on the midgut and sporozoites in the salivary glands. Wild-caught N. kolenatii were maintained on two wild-caught M. daubentoni harbouring heavy (patent) infections of P. murinus; both oocysts and sporozoites were found in these flies. The mature oocysts measured 52-71 micron in diameter. Sporozoites were straight or slightly crescentic and had a mean length of 7.4 micron. Electron microscopy of immature and mature oocysts revealed a morphology similar to that of malaria parasites. Sporozoites were also similar in structure to Plasmodium sporozoites and were found in the epithelial cells of the salivary gland and within the lumen; a cytostome was present and transverse sections revealed 21 microtubules arranged evenly around the periphery. Sporozoites were observed within the basement membrane of the salivary gland of N. kolenatii; such sporozoites appeared to be penetrating the gland, a process hitherto not described in malaria parasites. Rickettsia-like bodies were found within the cytoplasm of the epithelial cells of the salivary gland. Exflagellation of microgametocytes was achieved. An ultrastructural study of the gametocytes revealed a structure similar to that described in other Haemoproteidae. A common feature of infected erythrocytes was a projecting erythrocyte membrane. Attempts to find schizogony in impression smears and sections of tissues of two infected M. daubentoni were not successful.