During meiotic recombination in the yeast Saccharomyces cerevisiae, heteroduplexes are formed as an intermediate in the exchange process. In the formation of an asymmetric heteroduplex, one chromosome acts as a donor of a single DNA strand and the other acts as a recipient. We present genetic evidence that the nontranscribed strand is donated more frequently than the transcribed strand in spores that have an unrepaired mismatch at the HIS4 locus.