We have reported that a parental strain of Schizophyllun commune T11 (trp1) is fully compatible with a strain T37 (BK1PL4, trp1), but indole-and caffeine-resistant mutants (In -13 and Caf -9 respectively) derived from the T11 are semicompatible with the same T37. The compound-resistant mutations, ind1 and cfn1, were linked to neither A nor B mating-type factors. In -13, Caf -9, and all of indole-and caffeine-resistant progenies (ind1, trp1; cfn1, trp1) were semicompatible with the T37, and compatible with a strain T40 (BK1PL4, TRP1) although the T40 contained the same class III B mating-type factor as the T37. The In -13 and Caf -9 were also semicompatible with any tester strains (BK1PL4, trp1) developed, and the resulting heterokaryons produced non-aggregated and semiaggregated masses of hyphae, respectively. In the mutant heterokaryons, nuclei were distributed irregularly especially in case of [trp1/trp1] background ; anucleate, uninucleate, binucleate and multinucleate cells were found with modified hyphae which contained a different type of septation (pseudoclamps and simple septa other than true clamps). We concluded that the ind1 and cfn1 mutations alter the normal behavior of one of class III B mating-type factors in Trp 3 cells. z