The Co-Q system was investigated in thirty-one strains of the species of the genus Trichosporon and related organisms. Three Co-Q systems comprised of Q-8, Q-9, and Q-10 were recognized in these organisms. The Q-8 system was found only in T. behrendii, a synonym of Pichia burtonii [Endomycopsis burtonii]. The Q-9 system was distributed in T. aculeatum, T. capitatum, T. cutaneum, T. fermentans, T. penicillatum, T. pullulans, T. brassicae, T. sericeum [Endomyces ovetensis], and Geotrichum candidum. The Q-10 system was restricted in some strains of T. cutaneum. The taxonomic significance of the Co-Q system in the genus Trichosporon and related organisms was discussed.