A simple method is presented to measure the nutrient load of wastewater. Suspended particles, including microorganisms, are collected on a membrane filter by vacuum filtration, followed by colorimetric protein assay. Soluble proteins can be determined in the filtrate by precipitation. In domestic sewage and agricultural dairy wastewater, the largest portion of protein was retained on the membrane. Young larvae of Culex nigripalpus, Cx. quinquefasciatus and Aedes aegypti pupated when maintained in wastewater or its sediment but not when maintained in a membrane filtrate. If mosquito larvae feed primarily on suspended particles, the assay can be used to identify major feeding areas and may result in more economic application of microbial larvicides that act as food poisons.