Current concepts of biological control of plant pathogens have been developed using many of the classical studies of antagonism of Rhizoctonia solani Kuhn by Trichoderma spp. Most were descriptive, mycological, and physiological reports of the interactions. The objectives were to determine the function of R. solani, host plants, and Trichoderma spp. in the development of suppressiveness to R. solani in the soil ecosystem;R. solani and Trichoderma viride Pers.:Fr. were matched in dual systems in culture media and soils. Mycoparasitism occurred on highly deficient and complete media. T. viride growing from a complete medium was more parasitic on R. solani than when developing from a deficient medium. Hyperparasitism was greater where young hyphae of each fungus met than when T. viride encountered older hyphae of R. solani. R. solani induced rapid increases in Trichoderma spp. populations in soil and the stimulatory effect passed on air gap in an enclosed system. R. solani was cultured in beet seeds and introduced into the soil; only living R. solani in the seeds induced population increases of Trichoderma spp. R. solani infested seeds were parasitized by the hyperparasite, which displaced R. solani. Not all isolates of R. solani stimulated growth by Trichoderma spp; among 13 isolates only the four belonging to the AG-4 type, were highly stimulatory;Soil was infested or not with R. solani, planted with radish, cucumber, wheat (non-host), or not planted, and then replanted (or not) for six cycles. One unplanted treatment was reinfested with R. solani each cycle. Then all soils were infested with R. solani and planted to radish. All soils were somewhat suppressive to R. solani except the soil never cropped nor infested originally. Soil conduciveness was measured by the linear development of disease in radish planted radially around a R. solani pellet, and by weighing the soil aggregate formed by hyphae radiating from the pellet after 8 days. Adding R. solani inoculum to the soil each cycle with no cropping resulted in the least conducive soil; the most conducive one was never cropped nor infested with the pathogen. Infestation with R. solani resulted in less conduciveness than with no infestation, and conduciveness was greater with cucumber and wheat cropping and less with radish cropping. Suppressiveness was an additive trait that depended upon the frequency of R. solani activity in or additions to the soil. Development of suppressiveness did not require cropping.