The inhibition of ALA dehydratase by levulinic acid during ALA extracellular formation of Rhodobacter sphaeroides correlated with the concentration of undissociated form of levulinic acid irrespective of culture pH. The inhibition constant, Ki, of intracellular ALA dehydratase by Dixon plots was 2.95 mu M. Undissociated levulinic acid therefore functions as an inhibitor of ALA dehydratase.