Cell-wall-degrading enzymes are produced by fungal parasites. Parasitism of Rhizoctonia solani was first reported by Weindling (1932), who observed the parasitic nature of Trichoderma lignorum on several plant pathogens. Weindling’s (1932) report on the possible use of mycoparasites for the control of plant pathogens raised a great deal of interest in this area of research, including the role of hydrolytic enzymes in microbial interactions. It prompted reviews and discussions of the subject in numerous articles (Barnett, 1963; Boosalis, 1964; Barnett and Binder, 1973; Ayers and Adams, 1981; Wells, 1981; Baker, 1988; Ghaffar, 1988; Elad & Chet, 1995; see also chapter VI.B5). One of the features required for successful degradation of host fungi is the ability to produce the enzymes which degrade the polymers underlying the cell wall and membranes of the host. Boosalis (1956) estimated the frequency of R. solani parasitism by Penicillium vermiculatum in soil by observing the parasitism of R. solani hyphae in situ. Indeed, living fungal structures are often parasitized in natural soils (Boosalis, 1956; Hunter et al., 1977; Sneh et al., 1977). For this purpose, it may be assumed that the parasites produce the hydrolytic enzymes in natural niches where the pathogen is present. However, even with increasing evidence of natural mycoparasitism, the ecological significance of this phenomenon, at least in soil, is still ambiguous. Griffin (1972) maintained that mycoparasitism is of no ecological significance in soil, while others have suggested that mycoparasitism has considerable impact on soil fungal populations, by reducing the longevity of survival structures (Sneh et al., 1977). In any case, one of the features required for successful degradation of host fungi is the ability to produce enzymes which degrade the polymers underlying the cell wall and membranes of the host. The studies related to these enzymes, as related to degradation of R. solani, will be described in this chapter.