A free-living amoeba isolated from creek water could be grown at 24 °C. on a solid malt agar medium together with a pure culture of a motile Gram-negative bacillus, isolated from the same sample of water. The bacilli apparently served as the sole source of food for the trophozoites, the malt in the medium being-utilized by the bacilli for their own growth. In addition, the encysted form has been maintained on this medium at 24 °C. for the past 12 months and at 4 °C. for 10 months. Transfers to fresh medium of cysts from such cultures yielded numerous trophozoites after 12 hr. incubation at 24 °C, provided that metabolically active bacilli were present.