Bacteria that can degrade juglone (5-hydroxy-1,4-naphthoquinone) were isolated from soil beneath black walnut trees. Autecological studies with one of these bacteria (Pseudomonas J1), demonstrated that it could grow rapidly using juglone as its sole source of carbon and energy. Using nonlinear regression analysis and the Monod equation, it was determined that this bacterium had a high affinity for juglone (K s = 0.95 μg/ml).Pseudomonas J1 can also utilize other aromatic compounds from plants as its sole source of carbon and energy. Compounds such as chlorogenic acid, ferulic acid, gallic acid, and 2-hydroxy-1,4-naphthoquinone (Lawson) were rapidly degraded byPseudomonas J1. The rapid degradation of juglone and other suspected allelochemicals by soil bacteria make it unlikely that these compounds are important mediators of plant-plant interactions under natural conditions.