The accumulation of terpenoid aldehydes in seedling roots of four Upland cotton cultivars, Maxxa, Prema, Royale and GC 510, and two Pima cotton cultivars, S - 7 and OA 337, in response to infection by Pythium ultimum and Rhizoctonia solani was determined. The terpenoid aldehyde content of Pima cotton was initially higher than that of Upland cotton. Inoculation with both P. ultimum and R. solani stimulated terpenoid production in all cultivars with the exception of R. solani infection in the cultivar Prema. In Upland cotton, levels of terpenoids increased with plant age whether or not the roots were infected with a pathogen. There was no correlation between the accumulation of terpenoid aldehydes and the relative susceptibility of the cultivars to both P. ultimum and R. solani.