With three clones of Spironucleus muris (S. muris)--established from a mouse, hamster, and rat--homologous and heterologous host species were experimentally infected. Each host was susceptible to the clone originating from the homologous donor. In addition, both mice and hamsters were susceptible to the reciprocal heterologous clones. In contrast, infections of the rat with both heterologous clones were very poor, i.e. quantitatively low and ephemeral. It was not possible to infect hamsters and mice, not even athymic, with S. muris from the rat. This suggests a strain heterogeneity within the genus S. muris. In general, the genetic background of the host influenced the infection, the sex of the host did not.