An increase in the rate of ethylene production occurred in excised tomato roots, whether or not infected with Meloidogyne javanica, when incubated in media containing indole acetic acid (IAA) above 0·6 μM. However, the overall increase was considerably less pronounced in infected roots than in uninfected ones. Application of IAA to infected cultures resulted in an increase in gall fresh weight which was concentration-dependent and corresponded with the concentration of IAA, which stimulated ethylene production. The hypertrophied cortical parenchyma tissue in galls treated with 6·0 μM IAA was wider than that of the untreated galls. The levels of free and bound endogenous IAA in infected roots were higher than in uninfected tissues, attaining their highest levels 10 days after inoculation coincident with the time of the highest rate of ethylene production in the infected tissue. The interaction between ethylene and auxin regarding the host—parasite interrelationships of the root-knot nematode is discussed.