The nature of recombination modifiers was investigated in Bombyx mori lines selected for high (H) and low (L) recombination rates between the p(S) and Y loci in chromosome 2. Since the mean recombination rates for the H x L and L x H F(1) crosses were approximately intermediate between those of high and low lines, the cytoplasmic maternal effect and difference in the activity of recombination modifiers between marked and unmarked second chromosomes were not detected. The H x (L x H), H x (H x L), L x (L x H) and L x (H x L) backcrosses indicated the presence of additive and dominance effects of marked and unmarked second chromosomes and the remaining chromosomes.--Recombination rates between the p(S) and Y loci in chromosome 2 and half-nonrecombination rates between the pe and re loci in chromosome 5 of high and low lines indicated that these recombination modifiers caused changes in the recombination frequency between p(S) and Y in chromosome 2, but not between pe and re in chromosome 5.--There were no differences in viability between individuals having the second chromosomes of the recombinant types [p(S) +, p(Y) (H); p(S) +, + Y (L)] and those of the nonrecombinant types [p(S) Y, p + (H); p(S) Y, + + (L)] in both high and low lines. Mean recombination rates measured in cis [p(S) Y/p + (H); p(S) Y/+ + (L)] and trans [p(S) +/p Y (H); p(S) +/+ Y (L)] males were the same in the high but not in the low line. No segregation of a single recombination modifier was indicated by the distribution of recombination rates measured in trans males [p(S) +/p Y (H); p(S) +/+ Y (L)] of high and low lines. Accordingly, the recombination modifiers distributed on chromosome 2 in the heterozygous condition were not gross chromosomal aberrations, but polygenic factors in the low line.