Abstractδ‐Aminolevulinate (ALA) dehydratase is the second enzyme of the heme biosynthetic pathway. In mammals, ALA dehydratase is more active in the liver and erythroid cells than ALA synthase, the rate limiting step of heme synthesis in liver. Nevertheless, decreases in ALA dehydratase activity by chemical intoxication as well as by genetic disorders are known to suppress heme biosynthesis. In this report, we describe a) a comparison of the properties of ALA dehydratase from various species, b) gene activation of ALA dehydratase and the other heme enzymes during erythroid differentiation, c) the gene structure of ALA dehydratase and expression of mRNA encoding erythroid‐specific and nonspecific isoforms, and d) toxicologic and genetic mechanisms that may reduce ALA dehydratase acti