The herbicide simazine (2-chloro-4,6-bis(ethylamino)- 1,3,5-triazine) was found to have an inhibitory effect on nitrification in pure and mixed cultures of the nitrifying bacteria. Simazine concentrations of 6 and 9ppm inhibited the rate of nitrification in soil perfusion units, and an abnormally high level of nitrite nitrogen was observed in the herbicide treatments. The herbicide had no effect on the growth of Nitrosomonas europaea in shake flask culture, but did inhibit the growth of Nitrobacter agilis. The addition of yeast extract to the culture medium did not reverse the inhibition. Simazine did not inhibit the respiration of N. agilis even at a 100ppm concentration. Five organisms were isolated which could utilize simazine as a sole source of carbon and nitrogen. Pure cultures of these organisms were made and several were identified. / Master of Science