Biocontrol efficiency of four isolates of T. harzianum and T. viride were evaluated against twelve isolates of Rhizoctonia solani under greenhouse conditions.  R. solani and Trichoderma isolates were isolated from cotton seedlings.  Six of R. solani isolates belonged to AG4 and six belonged to AG2-2.  Analysis of variance showed very highly significant effects of Trichoderma spp. isolates, R. solani isolates, and their interaction on preemergence damping-off, postemergence damping-off, survival, plant height, and dry weight.  This interaction implies that a single isolate of antagonist can be highly effective against an isolate of R. solani, but may have only minimal effects on other isolates of Rhizoctonia solani.  The correlation among variables used for evaluating pathogenicity of R. solani isolates under effect of Trichoderma isolates was studied.  It was found that the application of Trichoderma as biocontrol agent changed the relationship between these variables.  Cluster analysis of Trichoderma isolates based on their antagonistic patterns showed that isolates were divided to two groups.  The first group included isolates of T. harzianum and isolate 2 of T. viride, while the second group included the other isolate of T. viride.  It seems that grouping of Trichoderma spp. isolates was not related to either geographic origin or0morphological taxonomy.  Cluster analysis of R. solani isolates based on their response patterns to Trichoderma isolates suggests that AG4 isolates were more homogeneous in their response patterns than those of AG2-2.