The effects of acidity and high levels of Al upon the growth of four strains ofBradyrhizobium japonicumwere compared in three strongly acidic soil extracts and in liquid defined medium. In all media the times for the strains to increase in cell density by a factor of 50,000 (i.e., become slightly turbid) increased as pH was decreased in treatments that included pH values of 6.7, 5.1, 4.8, and 4.6. Addition of 15 or 25 μM Al at pH 4.8 produced no detrimental effects in all media, whereas addition of 50 μM Al adversely affected the growth ofB. japonicumat pH 4.6. At a given concentration of total AlB. japonicumin general grew as well in defined medium as in soil extract media which would be expected to contain organic Al‐detoxifying agents. Growth curves indicated that the growth reduction by acidity and Al toxicity in the turbidity studies was the result of sustained decreased growth rates and not simply the result of an initial lag period.