As a freshwater formation, Reservoir Mulur is also a habitat for freshwater organisms. This includes a variety of plankton and benthos. The life of these organisms will be closely linked to environmental conditions, including water pollution levels. Sensitivity of aquatic organisms such as plankton and benthos make it as a good bio indicator for pollution. The purpose of this study was to determine the diversity level of plankton and benthos in Reservoir Mulur as well as to determine the pollution level of the waters of the reservoir. The analysis of pollution level is done by using the formula Shannon-Weiner diversity index. Water pollution level is determined by the amount of Shannon Weiner diversity index of plankton and benthos. The three sample locations of Reservoir Mulur generates Shannon Weiner ID values for plankton 2,219 (Station I), 2.427 (Station II) and 2.356 (Station III). As for the benthos Shannon Weiner ID values obtained 1,665 (Station I), 1.529 (Station II) and 0.975 (Station III). Based on the Shannon Weiner ID plankton, the entire site is being considered impure. This condition is different from the level of contamination based on Shannon Weiner ID benthos. The results based on a comparison of pollution levels in a row ID benthos were continuously polluted, moderate and severe.