Catalase and ascorbate peroxidase enzymatic activities were examined during the interaction between Nicotiana tabacum and the arbuscular mycorrhizal Glomus mosseae. Transient enhancements of both enzymatic activities were detected in the inoculated plant roots coinciding in time with the stage of appressoria formation in the root surface. The analysis of free salicylic acid content in roots revealed that the increases in enzymatic activities were coincident in time with the accumulation of SA in inoculated roots. These data indicate that the first reaction of the root cells to the invasion of arbuscular mycorrhizal fungi is a defence response.