To examine the relationship between genetic and physical chromosome maps, we constructed a diploid strain of the yeast Saccharomyces cerevisiae heterozygous for 12 restriction site mutations within a 23-kilobase (5-centimorgan) interval of chromosome III. Crossovers were not uniformly distributed along the chromosome, one interval containing significantly more and one interval significantly fewer crossovers than expected. One-third of these crossovers occurred within 6 kilobases of the centromere. Approximately half of the exchanges were associated with gene conversion events. The minimum length of gene conversion tracts varied from 4 base pairs to more than 12 kilobases, and these tracts were nonuniformly distributed along the chromosome. We conclude that the chromosomal sequence or structure has a dramatic effect on meiotic recombination.