A group of enzymes of sulfur metabolism (arylsulfatase, cholinesulfatase, and a number of others) are normally repressed in Neurospora crassa by an abundant supply of a "favored" sulfur source such as methionine or inorganic sulfate. A mutant called scon(c) was isolated in which the formation of each of these enzymes is largely or completely nonrepressible. The structural genes for three of these enzymes have been mapped; scon(c) is not linked to any of them. It is also not linked to cys-3, another gene which is involved in control of the same group of enzymes. Two alleles of the structural gene for arylsulfatase [ars(+) and ars(UFC-220)] produce electrophoretically distinguishable forms of arylsulfatase. Heterokaryons with the constitution scon(c) ars(+) + scon(+)ars(UFC-220) were prepared. These heterokaryons produce both forms of arylsulfatase under conditions of sulfur limitation, but produce only the wild-type (ars(+)) form under conditions of sulfur abundance. When the alleles of ars and scon are in the opposite relationship, only the ars(UFC-220) form of arylsulfatase can be detected under conditions of sulfur abundance. Thus the effect of the scon(c) mutation seems to be limited to its own nucleus. The implications of these findings are discussed.