Two new conversion control factors (ccfs), ccf-5 and ccf-6, have been characterized in the Pasadena strains of Ascobolus immersus. Both are monogenic, with two known allelic forms (called A and B) of each factor, and affect the frequency of meiotic gene conversion at a white (w) ascospore locus closely linked to it, ccf-5 affecting w-9 and ccf-6 affecting w-BHj. The ccfs appear to be specific to their own target site, with no effect on at least nine unlinked w mutations. Conversion of the w locus affected was studied in + × w crosses with all four possible ccf arrangements: for example, for + × w-9, with ccf-5(A) in both parents, with ccf-5(B) in both parents, with ccf-5(A) in +, B in w-9, and with ccf-5(B) in +, A in w-9. For both ccfs, there were slight differences between crosses homozygous for A and those homozygous for B, and also slight differences between the two forms of heterozygous cross, A/B and B/A, but the major effect was for heterozygosity for the control factor to depress conversion frequency of the w locus, compared with either homozygous state. These two ccfs are compared with other sites affecting recombination in fungi and higher eukaryotes. Two possible modes of action of ccfs 5 and 6 are (i) on pairing closeness before hybrid DNA initiation, and (ii) on later stages such as the spread of hybrid DNA from an initiation point.