The antibiotic effects of 3 phenolic substances viz. resorcinol, gallic acid and phloroglucinol on the cotton bollwormHeliothis armigera Hubner were evaluated with particular reference to the feeding regimes, survival and pupation. Some of the characteristic antibiotic effects observed for resorcinol and phloroglucinol include reduction in consumption index and approximate digestability, increased efficiency of conversion of ingested food and efficiency of conversion of digested food, extended larval duration and high mortality. However, there was no alteration in the various food utilization efficiencies in the case of gallic acid treated individuals. The defensive response of the larvae to these toxic substances seems to be excessive defaecation. Frass analysis through thin-layer chromatography and ultraviolet characterisation indicated the presence of the ingested antibiotics, opening further avenues of investigation on the possible role of these antibiotics present in the frass in biological control.