Our laboratory has been involved in a search for the active biochemical defense mechanisms which are under genetic control and which may account for the differing abilities of some Gossypium species to survive in the presence of the fungal wilt pathogen Verticillium dahliae. The mechanisms which have been demonstrated or suggested as important in the defense responses of cotton plants include phytoalexins, tannins, gums and tyloses, and lignin and suberin. Cotton phytoalexins, or antibiotics, are terpenes related to gossypol or cadalene and are products of the mevalonic acid (isoprenoid) biosynthetic pathway. The gossypol-related phytoalexins increase in total amounts and in the relative proportions of reduced forms in cotton stems in response to fungal infection. Cotton tannins (condensed proanthocyanidins) are strong protein denaturants, enzyme inhibitors and antisporulants, and are products of the shikimate (phenylpropanoid) pathway. Tannin levels increase in response to fungal invasion of the stele. Lignin polymers are also derived from shikimate and are widely demonstrated in various plant species as a physical defense against spread of vascular and other pathogens. Carbohydrate metabolism is the source of tyloses that are formed by the plant in response to invading vascular pathogens. There are also other defense mechanisms which we suspect remain to be demonstrated in the Gossypium system, such as chitinases to hydrolyse fungal cell walls.