The two yeast genes for 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase, HMG1 and HMG2, each encode a functional isozyme. Although cells bearing null mutations in both genes are inviable, cells bearing a null mutation in either gene are viable. This paper describes a method of screening for recessive mutations in the HMG1 gene, the gene encoding the majority of HMG-CoA reductase activity in the cell. This method should be applicable to the isolation of mutations in other cases of duplicated genes or duplicated functions in yeast. Four recessive point mutations were recovered in HMG1. These mutations exhibited intragenic complementation: one allele is in one complementation group and three alleles are in a second complementation group. Assays of HMG-CoA reductase activity indicated that the point mutations destroy most if not all of the activity encoded by HMG1. Intragenic complementation occurred with partial restoration of enzymatic activity. HMG1 was mapped to the left arm of chromosome XIII near SUP79, and HMG2 was mapped to the right arm of chromosome XII near SST2. A slight deleterious effect of a null mutation in either HMG-CoA reductase gene was detected by a co-cultivation experiment involving the wild-type strain and the two single mutants.