A brief description of 2 expts. in cultivating micro-organisms by continuously adding sterile substrate and removing the spent material and organisms. The rates of the 3 processes were so adjusted that the self inoculation is believed to have occurred at about the logarithmic growth phase of the organisms, thus the formation of mutants was inhibited. A description of the apparatus with diagrammatic sketches and suggestions for modification is included. The 2 organisms used in the expts. were a laboratory strain of Penicillium notatum and a commercial strain of brewer''s yeast.