Abstract 1. 1. Measurements were made of the growth in dry mass and volume of single cells of the fission yeast Schizosaccharomyces pombe. Dry mass was measured by an integration method with an interference microscope. Volume was measured from photographs. The yeast cells were grown in a gelled medium containing 17.5 per cent gelatin in order to reduce the phase retardation of the cells and to keep them still. 2. 2. The growth in dry mass was linear for each generation. Once a cell has been formed it maintains a constant mass growth rate until just before cleavage when the rate changes to that for the next generation. This suggests that synthesis is controlled by centres or particles, perhaps microsomal, which remain constant until division, when they normally double and partition themselves between two daughter cells. It is also suggested that, under mildly adverse conditions, the growth rate only changes at division, possibly because an insufficient quantity of a precursor has been made during the preceding generation. 3. 3. The growth in volume is different from the growth in dry mass. For the first three-quarters of a cell cycle, the volume growth rate slowly increases. This is followed by a constant volume stage in which there is no volume growth. The cell plate appears one-third of the way through this stage. 4. 4. The cell concentration, derived from the mass and volume figures, fluctuates cyclically, with a minimum at the start of the constant volume stage and a maximum at cleavage.