The proportions of arylsulfatases A, B, and B' differ markedly among various murine tissues. Although arylsulfatase B' may be a derivative of arylsulfatase B, their respective activities appeared to vary independently in the five murine tissues investigated. As-I, the apparent struc tural locus for murine arylsulfatase B, is expressed in brain, liver, kidney, lung, and spleen. Heat-stable arylsulfatase B is inherited as an autosomal incomplete dominant in most tissues. Current evidence suggests the existence of a regulatory element with cis-dominant ef fects that is situated near As-I and is expressed in these tissues. However, control of liver arylsulfatase B is subject to more complex control and may involve participation of several genes, some of which are unlinked to As-I.