The yield of prototrophic recombinants in crosses between nit-2 alleles is repressed in the presence of the rec-z+ gene. Similarly, recombination between his-1 alleles is repressed by the rec-1+ gene. Experiments reported in this paper show that if the rec-1+ and rec-z+ loci are different, the worst estimate indicates that they are located not more than O· 67 map units apart with 95 % probability. It is likely, therefore, that rec-1 controls the frequency of recombination between nit-2 alleles as well as between his-1 alleles. This is in line with evidence that each of the genes rec-2+ and rec-3+ repress recombination in more than one chromosomal segment. Nevertheless, each rec gene is specific in its action, repressing recombination in only a limited fraction of the genome.