Metasystox-R is a systemic soluble liquid insecticide for the control of aphids on brassica vegetable crops, cotton and lupins and it is possible enter to the marine environment and may be have a hazard effects for the marine organisms and nitrification processes. Effect of Metasystox-R on ammonia oxidizing activity by marine Nitrosomonas sp. was investigated by determining nitrification inhibitor assay in the cell suspension. Results showed that 8 microg mL(-1) of Metasystox-R with PI50 = 4.48 significantly inhibited nitrite production by marine Nitrosomonas sp. These results suggested marine Nitrosomonas sp. may be one of the target bacteria which was inhibitor and decreasing nitrification in the marine environment.