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In this paper we discuss some new uses for an old technique. The technique is the continuous culture of microorganisms, which in one form or another is familiar to every microbiologist. The particular continuous-culture device we will consider is the chemostat. A chemostat is basically a culture vessel having an input aperture for the influx of sterile nutrient medium from a reservoir and an overflow aperture for the efflux of exhausted medium, living cells, and cellular debris. The device (and the term" chemostat") was invented by Novick and Szilard (103); the" bactogene" is a virtually identical device developed independently and simultaneously by Monod (95). In practice, a chemostat apparatus is complicated by various attachments for aera-tion of the medium and for the prevention of contamination, and the overflow is often con-trolled by means of a siphon. The rate of cell division within the chemostat can be varied …