Atrazine and other pesticides may damage non-target ecosystems, such as wetlands, by destroying vegetation or by disrupting microbial communities and nutrient cycling. The addition of atrazine to a wetland microcosm will disrupt the nitrogen cycle by inhibiting nitrifying and denitrifying bacteria. The inhibition or absence of functional groups of bacteria will limit the ability of the wetland to remove nutrients and herbicides, thus increasing nitrogen levels in non-point source pollution. All tests were conducted in the laboratory using microcosms established from intact plant/sediment core subsamples collected from a natural freshwater wetland. Each microcosm received distilled water, atrazine, or acidified distilled water (pH 2) by groundwater seepage. Microcosms were dosed with 1.5 mg/L and 4.5 mg/L atrazine. Acid treatment of microcosms was done to provide a perturbation reference to the microcosm. Nitrite, nitrate, and ammonia levels were determined, as was dissolved oxygen, pH, and conductivity. The nitrifying bacteria and denitrifying bacteria were enumerated. Data were analyzed with one way analysis of variance (ANOVA) with block and treatment interaction and Tukey's Studentized Range test. Acid treatment of microcosms provided a positive control of microcosm disturbance. Acidification of the sediment disturbed the nitrogen cycle by suppressing ammonia dependent microbial processes. Atrazine (at 1.5 and 4.5 mg/L atrazine) did not significantly affect the number of nitrifying bacteria. The denitrifying or nitrate reducing bacteria were stimulated by exposure to atrazine and nitrate reduction became the dominant microbial process following exposure to atrazine. This shift toward denitrification and nitrate reduction processes caused increased levels of nitrite in the overflow waters. The continued accumulation of atrazine may cause nitrogen losses from an ecosystem which is already nitrogen limiting. Over time, predominant plant species may change, becoming primarily nitrogen fixing species. Changes in plant species would bring about changes in associated microbial populations. Excess nitrogen present in wetland runoff may cause algal blooms in receiving waters and pose health risks in contaminated drinking water. Accumulation of atrazine in wetlands may, ultimately, result in the loss of buffer zones. / Master of Science