Although no specific role has been demonstrated for ethylene during Arbuscular Mycorrhizal (AM) symbiosis, recent results suggest its participation in the regulation of the AM. Analysis of arbuscular mycorrhizal colonization in the abscisic acid (ABA)-deficient tomato sitiens mutant has shown that ABA deficiency induced ethylene production. It has also been suggested that one of the mechanisms used by ABA to determine susceptibility to fungal infection is negative modulation of the ethylene pathway. In this study, we describe the pattern of mycorrhization in mutant plants with altered ethylene biosynthesis and/or perception pathways. Epinastic (epi) plants with increased ethylene response were unaffected in terms of mycorrhizal frequency, although this mutation had a considerable negative impact on the intensity of mycorrhizal root colonization. The negative impact of the mutation in epi plants on the intensity of mycorrhizal root colonization was associated with a transitory increase in the transcript level of the LeETR6 ethylene receptor gene. On the other hand, ripenenig-inhibitor (rin) tomato mutant plants were positively affected in relation to all the mycorrhizal colonization parameters measured, suggesting that, at least in tomato plants, the regulation of AM formation is mediated by the RIN pathway.