Abstract Conidia of Trichoderma reesei QM9414 were treated with colchicine. Nuclei in colchicine-treated conidia enlarged. When the concentration of colchicine or the treatment time with colchicine increased, the diameter of nuclei became larger. Colchicine-treated conidia generated sectors on a medium containing benomyl. Some sectors formed many conidia or could not produce clear zones on the plate assay medium for cellulase production. According to the DNA assay of conidia, colchicine-treated strains were autopolyploid.