The chemically induced reversion frequencies of 77 mutants of Aspergillus nidulans were examined. The mutants which were at several loci concerned with nitrogen metabolism were induced by nitrous acid, N-methyl- N′-nitro- N-nitrosoguanidine and diepoxybutane and reversion was attempted by these same mutagens and diethylsulphate. The ability of the mutants to revert was affected by the inducing mutagen, the bifunctional diepoxybutane giving a much higher percentage of nonrevertible mutants than the monofunctional agenst, nitrous acid and nitrosoguanidine. 40% of the non-revertible mutants were associated with translocations and the bifunctional agent substantially increased the frequency of translocations over that given by the monofunctional agents. The frequency of reversion, among the individual revertible mutants, varied by a factor of more than 100 but even so inducing mutagen and type of mutant had a detectable influence on revertibility. Those mutants induced by diepoxybutane, that did revert, were most frequently by diepoxybutane itself though this agent was less effective at reverting mutants induced by nitrous acid and nitrosoguanidine. It is suggested that diepoxybutane can induce revertible mutants by a process other than a transitional type change, possibly by a transversional type change.