A total of 169 cross-transmission attempts has been made with 44 (11.8%) of the 372 named species of Eimeria of rodents. Of these, 161 were rodent-to-rodent, 6 rodent-to-lagomorph, and 1 each rodent-to-carnivore and rodent-to-bird. None of the last three categories was successful. In the rodent-to-rodent combinations, 39 (80%) of the 49 attempts to transmit a coccidian species from one rodent species to another of the same genus were successful, and only 14 (12.5%) of the 112 attempts to transmit a coccidium to a rodent of a different genus were successful. Eight of the successful attempts were with E. chinchillae, which was the only truly euryxenous species of Eimeria in the group. Two successful attempts were between between the closely related rodent genera Spermophilus and Cynomys, and two were both of E. separata from Rattus norvegicus to some genetic strains but not to others of Mus musculus. One attempt with E. vermiformis from Mus musculus to Rattus norvegicus required treatment of the rat with the immunosuppressant dexamethasone to succeed. More cross-transmission studies are needed to determine the host spectra of the species of Eimeria and other coccidian genera, and to determine the roles of genetics and immunosuppression in their transmission.