Mycobacterium chelonei, M. flavescens, M. fortuitum and M. sphagni were recorded in Sphagnum bogs, inhabited by L. rubicunda. Experiments with tritium labeled mycobacteria indicate that these are accumulated by the cladocerans. The L. ruhicunda larvae acquire the mycobacterian flora through ingestion of Cladocera. It is shown that a direct correlation exists between the mycobacteria concentration in the bog water and the larval growth of L ruhicunda.