Mycorrhiza resistant pea mutants (myc^-) provide a powerful tool for determining genes and cellular events essential to establishment of an arbuscular mycorrhizal symbiosis. In most of these mutants (myc^-1) the development of the fungus is stopped at the stage of appressorium formation on the root surface, whilst in others (myc^-2) root colonization occurs but arbuscule development is altered. Fungal and plant behaviour in pea mutants and their parent (myc⁺) is being compared in order to beter understand recognition events leading to establishment of a functional arbuscular mycorrhiza symbiosis. We review here cytological and biochemical studies of these mutants which provide evidence for the existence of symbiosis specific plant genes, and of their role in the control of defence responses and in recognition phenomena betwwen the symbionts.