Homothallic switching of the mating-type MATa gene in Saccharomyces cerevisiae results from replacement by gene conversion of MAT-Ya DNA with Y(alpha) sequences copied from one of two unexpressed donors. MATa preferentially recombines with HML(alpha), located near the left end of chromosome III, but can use HMR(alpha), near the right chromosome end. MATa donor preference depends on a 700 bp orientation-independent cis-acting recombination enhancer, located 17 kb proximal to HML. Deletion of this element markedly reduces MATa's use of a donor inserted at any of four different locations along the leftmost 92 kb of chromosome III. This enhancer is sufficient for donor activation, since it stimulates use of the "wrong" donor, when it is inserted 7 kb proximal to HMR.