Mixed-genotype infections occur frequently in natural populations. Parasite genotypes are expected to interact within a host: competing for shared nutrients and being affected by the host's immune response to each other. Theoretically, competing parasites can be expected to exhibit increased rates of replication. Here, we investigate whether interactions between clones of Theileria annulata, a protozoan parasite of cattle, affect clones' replication rates in mixed cultures in vitro. Intrinsic replication rates and carrying capacities estimated from single-clone control cultures were used to predict replication rates of mixed cultures under different competitive assumptions. Mixed-culture dynamics deviated significantly from expectations in five out of six different clone combinations tested. Contrary to expectation, mixed cultures often replicated more slowly than predicted from single-clone control cultures. Competition coefficients were calculated from the mixed-culture data and a competitive hierarchy of clones determined. The results suggest that inherent competitive ability may be greater in clones with lower carrying capacities-those clones which would otherwise be excluded in a genetically diverse environment. Moreover, significant negative deviations from expected replication rates corresponded with successful out-competing of a higher carrying capacity clone by a lower carrying capacity clone.