We describe a laboratory model using Eimeria falciformis and E. pragensis to investigate some of the interactions in double-species infections of Eimeria. Mice were given trickle infections by oral inoculation of 100 sporulated oocysts of one species at 3 or 4 day intervals throughout the experiments and, once immunity had developed, as indicated by cessation of oocyst production, the animals were challenged with a single inoculation of the other species. A trickle infection of E. falciformis gave a significant enhancement of oocyst output from E. pragensis infection as compared with animals that had not received E. falciformis. Histological examination of the mice infected with E. pragensis showed significantly more asexual parasites and a significantly higher female/male ratio in the mice that had received trickle E. falciformis infections than in those that had not. There was no evidence to suggest that extra asexual generations were occurring in these mice. In the converse experimental protocol, trickle E. pragensis infections had no significant effect on a single E. falciformis infection. We discuss the possible mechanisms for the interactions and also how these interactions may influence multiple species infections in animals in their natural habitats.