Growth and symbiotic N2 fixation of peanuts (Arachis hypogaea L.) and other legumes were evaluated in three glasshouse experiments using soil treated with aluminium sulphate [Al2(SO4)318H2O] and calcium oxide (CaO) to produce various levels of pH and Al content. Nodulation and N2 fixation of peanuts and cowpeas [Vigna unguiculata (L.) Walp.] were inhibited when grown in soil (pH 3.8 and 3.9) treated with 2 mEq Al2(SO4)318H2O 100g -1 soil or its (SO4)3 equivalent of 960ppm sulphuric acid (H2SO4). Shoot growth was also drastically reduced and root damage typical of Al toxicity was observed. As pH was increased by CaO treatments, shoot and root growth, nodulation and N2 fixation improved. Maximum peanut root growth occurred at pH 7.3. Shoot growth, nodulation and N2 fixation of peanuts and cowpeas were best at a pH 5.9-6.3. Shoot and root growth of peanuts was relatively higher than that of cowpeas at pH 3.8. Shoot and root growth of mungbean [Vigna radiata (L.) Wilczek] and pigeon pea [Cajanus cajan (L.) Millsp.] was less than that of peanuts and cowpeas and pH 3.9. Nodulation and N2 fixation were higher at pH 3.9 in cowpeas than in peanuts and other legumes.