Soil cultures, enrichment cultures, and pure culture isolates produced substantial quantities of salicylic acid from naphthalene in a mineral salts medium containing NH(4)Cl as the nitrogen source. However, when KNO(3) was substituted for NH(4)Cl, these same cultures failed to accumulate detectable quantities of salicylic acid but did turn the medium yellow. When an isolate identified as a Pseudomonas species was used, viable cell numbers were much greater in the medium containing KNO(3), but up to 94% of the naphthalene was utilized in both media. After 48 h of incubation in a 0.1% naphthalene-mineral salts medium, the cultures containing NH(4)Cl showed irregular clumped cells, a pH of 4.7, 42 mug of salicylic acid per ml, and the production of 4.4 ml of CO(2). Under the same conditions, the cultures in the medium containing KNO(3) showed uniform cellular morphology, a pH of 7.3, no salicylic acid, the production of 29.7 ml of CO(2), and a distinct yellow coloration of the medium. The differences between nitrogen sources could not be accounted for by pH alone since results obtained using buffered media were similar. Growth with NH(4)NO(3) displayed a pattern similar to that obtained when NH(4)Cl was used. The yellow coloration in the medium containing KNO(3) was apparently due to more than one compound, none of which were 1,2-naphthoquinone or acidic in nature, as suggested by other investigators. Further attempts to identify the yellow compounds by high-pressure liquid chromatography, infrared analysis, and gas chromatography-mass spectrometry have been unsuccessful thus far.