The amino acid composition of the cytochrome b gene of 38 fungal and 12 oomycete species from the NCBI database was analyzed to identify key polymorphic sites at the Qi center of the proton-motive Q-cycle. The objective was to determine whether amino acid substitutions reflect a significant variation at the Qi target site among true fungi and oomycetes. The analysis was directed mainly to 3 amino acid changes N31K, G37C and L198F, which were reported elsewhere to be associated with low sensitivity of Saccharomyces cerevisiae mutants to the Qi inhibitors UK-2A, antimycin A and fenpicoxamid (Inatreq™ active). The fungal and oomycete species analyzed contained wild type amino acid variants at the 31, 37 and 198 positions except Rhizoctonia solani which carried the amino acid change L198F. Sequencing of the R. solani cytochrome b gene confirmed the presence of phenylalanine at the 198 position. An in vitro sensitivity test showed that this R. solani isolate was fully resistant to UK-2A, but remained sensitive to azoxystrobin, fluxapyroxad and epoxiconazole. The relationship between the change L198F and the low sensitivity of R. solani to UK-2A remains to be determined.