Nitrogen cycle in the biosphere depends on the processes of biological nitrogen fixation, nitrification and denitrification, and among the external factors that can affect them, the release of pesticides to the environment may play a major role. Denitrification (dissimilatory nitrate reduction, nitrate respiration) is characterized by consecutive steps starting from nitrate via  nitrite, nitric oxide (NO), nitrous oxide (N₂O) to dinitrogen gas (N₂). Whereas N₂ is an inert and harmless product, NO₂^-  and N₂O are a source of concern in our environment.  Nitrite  may reacts with various chemical to form toxic substances, and nitrites alone have been to be toxic to plants, animals and humans. Nitrous oxide is of concern because its release from soil to the atmosphere might cause damage of the stratospheric ozone layer. In this sense, several investigations have been carried out on the effects of pesticides on the survival and biological activity of soil denitrifying bacteria, but little is known about the effects of pesticides on the N₂O released and NO₂^- accumulation by denitrifying microorganisms both in soil microcosms and in pure culture. In this review, we examined the effect of application of pesticides commonly used in agriculture on the denitrification  process, and summarized aspects of the current knowledge about the ecology of denitrifying  microorganisms.