Proteins of 15 isolates of T. longbrachiatum and T. harzianum were compared by polyacrylamid gel electrophoresis (PAGE) and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Gels were stained with silver nitrate. Protein banding patterns of the isolates were subjected to cluster analysis by the unweighted pair-group method based on arithmetic mean (UPGMA). Apart from T. harzianum isolates T6 in PAGE and T27 in SDS-PAGE, which occupied separate positions in the phenograms, it was possible to delineate the isolates of each species on the basis of the results of PAGE and SDS-PAGE. Peroxidase (EC 1.11.1.7) and esterase (EC 3.1.1.1) isozymes from mycelium and conidia of Trichoderma isolates were separated by PAGE, and the obtained banding patterns were visualized by using specific staining systems. Cluster analysis indicated that delineation of the isolates of each species was not possible on the basis of the electrophoretic patterns of peroxidase. On the contrary, patterns of esterase isozymes provided a reliable method for grouping the isolates of each species with the exception of T9 and T14 of T. longbrachiatum, which occupied separate positions in the phenogram.