Interactions between parasite genotypes sharing a host are poorly understood, but have important consequences for the epidemiology and evolution of the parasite. In mixed-genotype malaria infections, patterns of asexual replication and transmission favoured by natural selection may be different from those in single-genotype infections. The infectivity to mosquitoes of mixed-genotype and single-genotype infections were compared using 2 clones of Plasmodium chabaudi inoculated into mice either together or alone. Mice given mixed-clone infections received the sum of the inocula given to the single-clone controls. Mosquitoes were fed on the mice and the numbers of oocysts which developed were counted to assess transmission intensity. For 3 combinations of starting inocula and feed days, mixed-clone infections produced more oocysts per mosquito than the sum of the 2 single-clone infections. This effect was correlated with an increase in gametocyte density, but was less clearly related to asexual infection parameters. The results show that interactions between clones in mixed-clone infections can profoundly affect transmission.