The spatial and temporal distribution of Chromobacterium in rivers was studied using a selective medium developed for the routine enumeration of small populations of these organisms. These studies gave a strong indication that numbers of chromobacteria were highest during winter months. This correlation was, at least in part, ascribed to increased carriage of silt by the river, derived from washed-in soil and resuspended sediment, during these months of high surface run-off and, consequently, high river flow-rate. The distribution of chromobacteria in soils and river sediments was investigated and was found to be widespread but irregular, and could not be correlated with any particular property of the deposit. In laboratory experiments, chromobacteria were found to proliferate in river water at 6°C (temperature equivalent to that of the river in winter), but rapidly disappeared at 15-17°C (equivalent to the river in summer). Chromobacteria seemed to be widespread in rivers, being isolated from all rivers sampled. Counts of chromobacteria were low from young, clear streams, but increased as the river aged. Through a comparison of the properties of strains of Chromobacterium, some strains from soil were found to be indistinguishable from most river and sediment isolates, while other "gelatinous" types were considered as characteristic of soil. A distinctive group producing thin spreading colonies were isolated only from water, and although psychrophilic, they were clearly separated from the accepted concept of Chromobacterium lividum.