Summary Malic enzyme or malate dehydrogenase (NADP +). E.C.I.I.I.40. catalyses the reaction: L‐malate + NADP⇄2 pyruvate + CO2+ NADPH Baker & Manwell (1977), in a survey of a number of different enzymes reported that ‘malic enzyme’ was polymorphic in the erythrocytes and certain other tissues of sheep, and indicated that it would be a potentially useful new genetic marker for this species. This paper confirms the existence of the polymorphism in sheep erythrocytes and presents inheritance and breed data.