The mating system of Coprinus macrorhizus f. microsporus was analyzed, using two stocks X and d. As shown in Table 1a, in matings between monosporous mycelia having unlike incompatibility factors at both loci, clamp-bearing hyphae are observed not only in the contact zone between two mated mycelia but also on both sides of it (complete dikaryotization). Clamps are also found in all matings where theB-factors but not the A-factors are identical (common B-factor mating). In the latter case, however, the formation of clamps is restricted only to the contact zone (limited dikaryotization). Therefore, when only the contact zone is examined for the presence or absence of clamps, a bipolar mating-pattern is obtained. However, when the mycelia on either side of it as well are tested for clamps, tetrapolarity is unmasked. Such tetrapolarity may be called “masked tetrapolarity”. All matings where hyphae with clamps had been observed were tested for their capacity to produce fruit-bodies under the same culture conditions. As shown in Table 1b, perfectly developed fruit-bodies with abundant basidiospores were obtained not only in all matings showing complete dikaryotization but also in some pairings showing limited dikaryotization. Fruit-bodies from the former matings produced spores of all four mating types ; whereas, from fruit-bodies formed in the latter pairings, only spores of the two parental types were produced (Table 2). Pairings between the monosporous mycelia of illegitimate origin show bipolar pattern, where only limited dikaryotization, but never complete dikaryotization, regularly occurs (Table 3a). In common B-factor matings, when two mycelia are inoculated 1-2cm. apart, a clear line of demarcation which is called “barrage” always appears between them, as shown in Fig. 1; whereas, when the two inocula are brought into contact with each other, a sector composed of dikaryotic mycelium develops occasionally in some matings, as shown in Fig. 2. Barrages develop with regular manner in all common B-factor matings, but the sectoring dikaryons are rather of haphazard occurrence.