Colonization of roots by arbuscular mycorrhizal (AM) fungi and the formation of structures that characterize the AM symbiotic association, i.e. appressoria, extraradical and intraradical hyphae, arbuscules and vesicles, involve a complex series of events. Plant mutants that show a block at some stage in the colonization process are useful in determining the factors involved in the interaction between hyphae and root cells at each step in that process. Most of the mutants identified to date are legumes that also show some impairment in nodulation. There is a tight correlation between the degree of impairment in nodulation and AM formation. For example, failure to initiate nodules (nod^-) in several legume species is associated with the failure of roots to be colonized by AM fungi (myc^-). Mycorrhiza formation in these mutants is blocked at the stage of appressorium formation and involves chemical and structural changes in the root epidermis. Colonization of other mutants by mycorrhizal fungi is blocked at other stages of mycorrhizal development. Although the molecular basis for the various steps in nodulation has been worked out in detail, there is less information for the AM symbiosis. It has been suggested, based on some experimental evidence, that these symbioses may share some common genes. A mutant of a non-legume species, tomato, has been isolated recently that shows various degrees of reduction in AM formation depending on the fungus species used as inoculum. Mutants of additional non-legume species are needed for the study of the developmental regulation of this symbiosis since the majority of AM fungus host plants are non-legumes.