Resistance of cotton breeding lines to Verticillium dahliae is correlated to the toxicity of the phytoalexin composition. Kenaf (Hibiscus cannabinus L.) produces the phytoalexin hibiscanone which is a 1,2-naphthoquinone. Hibiscanone is significantly more toxic to V. dahliae than desoxyhemigossypol (dHG), the most toxic phytoalexin produced by cotton. It may eventually be possible to transfer genes from kenaf that would produce 1,2-naphthoquinones in cotton. Before genetic studies are initiated, it is appropriate to determine which, if any, 1,2-naphthoquinones that might be produced in cotton would be the most effective phytoalexins. We have synthesized o-hemigossypolone (a 1,2-naphthoquinone related to hemigossypol) and some 1,2-naphthoquinone analogs. We found that 6-methoxy- and 7-methoxy-1,2-naphthoquinones are more toxic than dHG to V. dahliae. However, o-hemigossypolone, and 6-hydroxy- and 7-hydroxy-1,2-naphthoquinones are significantly less toxic. Thus, the random introduction of foreign genes that affect phytoalexin structure could produce a cotton more susceptible to V. dahliae. Any target molecule must be carefully evaluated prior to genetic incorporation into cotton.