Hydrogen peroxide (H 2 O 2 ) tolerance of Rhodococcus sp. strain APG1, previously isolated from the aquatic fern Azolla pinnata, was examined in relation to nitric oxide (NO) production by cells cultured on a variety of C sources. Cells inoculated onto A. pinnata fronds established a surface-sterilant resistant density of 2-4x10 7 cellsg - 1 without causing disease. Compared to cultures containing glucose, fructose, mannitol, or glycerol, those provided only with sucrose displayed, on a per C basis, substantially lower (<10%) growth yields and higher resistance to H 2 O 2 . NO, a positive regulator of catalase synthesis in bacteria, was produced in larger amounts in sucrose-grown cells as evidence by eightfold greater per cell accumulations in the medium of nitrite (NO 2 - ), a stable oxidation product of NO. Addition to cells of l-arginine, the substrate for nitric oxide synthase (NOS), stimulated production of NO, detected both by fluorometric reaction with diaminofluorescein-FM diacetate (DAF-FM DA) and by increased levels of NO 2 - in the culture medium. These results suggest that sucrose may enhance H 2 O 2 tolerance of Rhodococcus APG1 by increasing cellular NO producing capacity. We propose a regulatory role for NOS in promoting tolerance of Rhodococcus APG1 to oxidative stress in the phyllosphere.