Yeast mutants defective in beta-hydroxy-beta-methylglutaryl-CoA synthase and acetoacetyl-CoA thiolase have been isolated. Mutants impaired in acetoacetyl-CoA thiolase range into two linked complementation units, erg 10 A and erg 10 B. Mutants deficient in beta-hydroxy-beta-methylglutaryl-CoA synthase belong to two unlinked complementation groups, erg 11 and erg 13. In strictly anaerobic growth conditions, mutants impaired in beta-hydroxy-beta-methylglutaryl-CoA synthase require mevalonic acid in addition to sterol and oleic acid, pointing out the role of mevalonic acid in other physiological function than ergosterol precursor. Growth of mutants impaired in acetoacetyl-CoA thiolase cannot be recovered by mevalonic acid supplementation, suggesting a role of acetoacetyl-CoA or thiolase not linked to sterol pathway.