A rapid and sensitive photometric method was devised to assay naphthalene dioxygenase in whole cells of Pseudomonas fluorescens NCIMB 40531, a strain derived from a naphthalene-metabolizing isolate by means of N-methyl-N’-nitro-N-nitrosoguanidine mutagenesis. The naphthalene-assimilating pathway of NCIMB 40531 is functionally blocked at the level of cis-1,2-dihydroxy-1,2-dihydronaphthalene dehydrogenase and therefore cis-1,2-dihydroxy-1,2-dihydronaphthalene (naphthalene dihydrodiol) is accumulated when cultures are supplied with naphthalene. This modified metabolism allowed dioxygenase to be assayed by monitoring product formation. Optimal conditions were selected to give linear optical density vs time curves and reaction rates proportional to dry cell weight (DCW) : specific activities of 0.125(+/-0.008) mu mol.min(-1).mgDCW(-1) were consistently obtained in cultures grown on succinate in the presence of naphthalene as inducer. By means of the developed assay, 62 compounds (mainly mono- and bicyclic aromatics) were screened as potential inducers of the dioxygenase activity, when added to the growth medium at the concentration of 100 mg.l(-1) : besides naphthalene, the highest activities were induced by 3-methylsalicylic acid (2-hydroxy-3-methylbenzoic acid), O-acetylsalicylic acid and 5-chlorosalicylic acid with 0.198, 0.167 and 0.076 mu mol.min(-1.)mg DCW-1, respectively. Under the conditions used, no detectable dioxygenase activity was induced by salicylic acid, which is recognized as the natural inducer of the enzyme in Pseudomonas.