The effects of the application of eight pesticides (aldrin, lindane, dimetoate, methylparathion, methidation, atrazine, simazine, and captan) on growth, respiratory activity (as CO2 production), denitrifying activity (as N2O released), and nitrite accumulation in the culture medium by Paracoccus denitrificans strain ATCC 19367 were studied. The fungicide captan totally inhibited growth and biological activity of P. denitrificans, while the rest of the tested pesticides delayed the growth and CO2 release of P. denitrificans but did not drastically affect the bacterial growth or respiratory capacity after 96 h of culture. The denitrifying activity of P. denitrificans ATCC 19367 (as N2O released) was negatively affected by all tested pesticides. The release of N2O was strongly inhibited by several organochlorinated and organophosphorated insecticides (aldrin, lindane, dimetoate, and methidation), which led to high accumulation of nitrite in the surrounding medium. Atrazine decreased N2O release after 48 h of culture because of negative effects on growth, and methylparathion and simazine delayed the onset of N2O release by P. denitrificans. These three pesticides reduced the accumulation of NO2- compared to unamended control cultures.