Two uridine auxotrophic mutants of Trichoderma reesei were isolated by resistance to 5-fluoroorotic acid after UV mutagenesis.One mutant,called M23,was complemented with the Aspergillus niger pyrG gene carded by plasmid pAB4-1.A mutated pyrG gene of M23 was cloned and DNA sequencing analysis indicated that a cytosine was inserted into the 934-939 oligo dC position of the pyrG coding region,resulted in a frameshift mutation.Transformation efficiency was approximately 200-300 transformants per microgram of DNA with plasmid pAB4-1.Stable transformants were obtained by monosporic culture and showed to be prototroph after successive propagation.Vitreoscilla hemoglobin expression plasmid pUCVHb was cotransformed with plasmid pAB4-1 and attained a transformation efficiency of 71.8% or of 26.1% with pAN7-1.Southern blot analysis of the transformants demonstrated that plasmid pUCVHb was integrated into the chromosomal DNA.The experimental results demonstrated that the pyrG-based system was more efficient and timesaving than the conventional hygromycin B resistance-based transformation system.