Twelve cotton defence-response genes were analysed to determine if there is any similarity between the response of the plant to fungal pathogens or symbionts. The defence-response genes representing the enzymes phenylalanine ammonium lyase (PAL), chalcone synthase (CHS) and chitinase did not appear to be activated in mycorrhizal cotton roots, in contrast to what is observed in other plant-mycorrhiza interactions. Other defence-response genes with unknown function which were activated in myc01Thizal cotton roots deserve further investigation. One which has sequence similarity with a putative signal transduction protein may provide insights into the mechanism by which the defence response is regulated in mycorrhizal cotton roots.