A fungal strain, C-4, was isolated from etiolated leaves. Based on taxonomic studies, the fungus C-4 can be classified as a strain of Trichoderma species. When strain C-4 was cultured in Mandels' medium at 28degreesC for 6 days, the enzyme activities detected in the broth corresponded to 8.2 U/ml (28.1 U/mg) carboxymethylcellulase activity. An endoglucanase (EG; F-I-II) was purified from the culture filtrate of the strain through a four-step procedure-chromatography on Sephacryl S-200, DEAE-Sephadex A-50, Con A-Sepharose, and Chromatofocusing on Mono-P (HPLC). The molecular weight of this EG, which was called C4endoII, was determined to be about 51 kDa. The optimum temperature and pH of C4endoII were 50degreesC and 5.0, respectively. Incubation at 50degreesC for 24 h did not destroy the cellulose degradation activity. Amino acid sequence analysis revealed the N-terminal sequence of an internal peptide of C4endoII to be Phe-Ala-Gly-Ile-Asn-Ile-Ala-Gly-Phe-Asp-Phe, which is homologous to EGII from Trichoderma reesei. A C4endoII cDNA (C4endoII) was cloned from a cDNA library constructed using the mRNA of the strain cultivated in a cellulase-induction medium. The deduced protein sequence of C4endoII was 417 amino acids long and had a putative signal sequence of 21 amino acids with a predicted cleavage site after Ala-21. A single potential N-glycosylation site was present in the amino acid sequence.