In agreement with the results obtained in Escherichia coli by other workers and our own previous data, the kinetics with which spontaneous mutations to resistance to the 12,13-epoxytrichothecene trichodermin accumulate in a lysine auxotroph of Schizosaccharomyces pombe are dependent upon the nutrilite used to limit the growth of the population. Under conditions of glucose-limitation mutation accumulation is proportional to generation time, while under lysine-limitation it becomes proportional to chronological time. In contrast to observations made in bacterial system, however, no significant change in the DNA content per cell is noted in slow growing cultures grown under amino acid starvation. These findings help to eliminate some of the theories put forward to explain the differential mutational responses observed under different growth limiting regimes.