Each of the main laboratory wild stocks of N. crassa carries one of two alleles at the rec-1 and rec-2 loci and one of three at the rec-3 locus. The constitutions of the stocks are given in Fig. 1. Some of those conserved are evidently not the originals. The third rec-3 gene (rec-3L), found in Lindegren A, controls recombination at the am-1 locus to a level between that of rec-3+ and rec-3, the relative levels being 1 : 8 : 25. At the his-2 locus rec-3L is indistinguishable from rec-3+ in its level of control. This proves that there are minor differences between the control (con) genes, near to am-1 and his-2, which recognize products of rec-3 genes. Further, this is the first clear evidence, though indirect, that the binding sites for products of rec genes are situated in the chromosome regions where recombination is modulated.