3′,4′-Dichloropropionanilide; its biodegradation products 3,4-dichloroaniline and 3,3′,4,4′-tetrachloroazobenzene; and related chloroaniline and chlorobenzenes were tested for effects on the back mutation frequency of the meth3 locus in Aspergillus nidulans. 3,4-Dichloroaniline was more mutagenic than 3,3′,4,4′-tetrachloroazobenzene and 3′,4′-dichloropropionanilide. Chlorine in the para position on benzene ring increased mutagenicity more than chlorine in the meta or ortho position. Chloroaniline was more mutagenic than the corresponding dichlorobenzene.