Bidirectional selection was performed for increased and decreased recombination frequency (RF), between the ascospore colour gene asco and the centromere in linkage group VI of Neurospora crassa. In the selection scheme rather few individuals were selected in each generation, but optimal reassortment of recombination-controlling factors was pursued by formation of all possible combinations between the individuals crossed to create the next generation. After six generations, RF had changed significantly, from 12 % in the base population to 27 % in the high line and 8 % in the low line. The regression lines for the generation means on generation numbers were significant at the .01 level for selection in both directions. The genetic variance decreased rapidly, presumably due to the low number of parents used in each generation, and was almost exhausted in the sixth generation of the high line and even earlier in the low line. By detailed analyses of the material under selection and by reciprocal crosses with material from the sixth generation, it was demonstrated that additively acting chromosomal factors and maternally inherited factors, as well as epistasis and dominance, all influence the RF in the interval studied. In particular, high RF to a great extent was governed by maternally inherited factors, whereas low RF was not.