14C-Labeled pencycuron was applied to four strains of Rhizoctonia solani different in the sensitivity to this fungicide. Of the four strains two were sensitive (Sl=C-423, AG***-1, and S2=R-C, AG-4) and the other two tolerant (Ti=Rh-13 1, AG-4 and T2=SH-1, AG-5) to pencycuron. Strain S 1 metabolized 85% of the pencycuron in a 0.5ppm liquid medium, while strains S2, T1 and T2 slowly metabolized only 10-20% of the pencycuron 24 hr after incubation. Main metabolites were cis and trans-3-hydroxycyclopentyl pencycuron (P-1 and P-2, respectively), whose fungicidal activity was weaker than pencycuron. This suggested that pencycuron itself is an ultimate active substance, and that metabolic activation or detoxification is less important to explain the difference among the strains in the sensitivity to pencycuron. * Thanatephorus cucumeyis (FRANK) DONK is adopted for the perfect stage. *** Anastomosis group.