Schizosaccharomyces pombe mutants have been selected on the basis of an altered response to nutritional stimulation of cell division (changed division response, cdr). Two new loci (cdr1 and cdr2) were identified and characterized. When suspended in nitrogen-free medium wild-type cells underwent stimulated rates of division and became reduced to approximately 30% in protein content with a concomitant 3.6-fold increase in cell number after 24 h starvation. cdr cells had significantly smaller increases in cell number. The ratio of starved/unstarved protein content was higher for the cdr strains than for the wild type. cdr cells were also affected in their response to nitrogen-source shifts from proline to glutamate (or vice versa) or when shifted from serine phosphate to inorganic phosphate, showing that the alteration in division response was not restricted to nitrogen metabolism. Upon nitrogen starvation wild-type cells arrested prior to the cdc10 execution point, whereas cdr cells arrested later in the cell cycle. cdc25-22 cdr1 or cdr2 double mutants grew very slowly and were extremely elongated at all temperatures; the restrictive temperature was reduced to 27 degrees C. wee1 was epistatic to cdr mutations with respect to cell length at the cell plate stage. cdr+ genes are postulated to play a role in the nutritional modulation of the mitotic size control.