Horizontal transmission testing with an Amblyospora species from the mosquito Culex salinarius has documented the involvement of a copepod intermediate host. Meiospores (one type of uninucleate spore) of the Amblyospora sp. were infectious per os to female Macrocyclops albidus adults. All developmental stages in the copepod had unpaired nuclei (were haplophasic), starting with the sporoplasms from the meiospore, continuing as a succession of schizonts undergoing binary division and ending with sporulation, producing a second type of uninucleate spore. These spores, formed in the ovaries of M. albidus, were lanceolate, slightly curved and measured 13.23 x 3.85 mu m. They infected C. salinarius larvae, both male and female, when ingested. In addition, cross-infectivity testing was conducted and demonstrated that while A. californica from C. tarsalis will infect C. salinarius, it does not complete its life cycle in this host. Based on these findings, we conclude that Amblyospora sp. from Cuter salinarius is a distinct species and assign it the name Amblyospora salinaria n. sp. (C) 1998 Academic Press.