Aspects of orchid/endophyte relationships were studied "by investigating penetration and infection in clonal materials of C.yinbidium and Dendrohium infected with various Rhizoctonia isolates. .The changes in the orchid tissues after infection were also studied. In infection studies, it was found that Cym'bidium protocorms formed a symbiotic relationship with several Rhizoctonia strains while Dendrohlum protocorms were parasitised and died when infected with any of the Rhizoctonia isolates used in this work, Rhizoctonia isolate T (obtained from Dactylorhiza pur-pure 11a roots) was found to be a good endophyte for CynVbidium. Environmental factors such as; temperature, light and nutrient levels affect the penetration and infection of the orchid protocorms. In particular, infection of Cyitfbidlum protocorms was more rapid on a medium containing cellulose than on a dextrose medium. The incubation of Cyrubidium protocorms in light resulted in differentiation of leaf and root. The pelotons were formed 5 days after infection and the digestion completed days after that. Infected Cyrnbidxum protocorms released an antifimgal substance(s), which inhibited the germination of Botrytis cinerea spores, and this could not he detected in non-infected protocorms. As a result of infection, the respiration and enzyme activities (Polyphenol oxidase, Ascorbic acid oxidase, Peroxidase, and Catalase) were found to increase compai'ed to those in non-infected protocorms. This was found to he correlated with the formation and digestion of pelotons. The chlorophyll content of infected Cymbldium protocorms was more than those of the non-infected ones. The increase in metabolic activity reached a maximum nine days after infection (in cellulose medium) and. then declined slowly. A secondary rise in respiration was observed in some C.ymbidium protocorms, which was attributed to a secondary infection leading to the formation of new pelotons. The changes in metabolism in orchid protocorms were similar to those reported for infected non-orchid hosts.