Yield of the supra-littoral chrysomonad Monochrysis lutheri varied directly as vitamin B12 concentration between the limits 0·1 and 100 μμg/ml., each μμg of vitamin supporting 0·55 × 106 cells. Between the same limits of concentration there was to be seen no significant deviation in initial rate of cell division from the mean value of 1·1 divisions per day (phototrophic growth: 2000 lux, 20° and 15° C), the conclusion being that limitation of the division rate does not occur before the concentration vitamin B12 has dropped below 0·1 μμg/ml.