In response to the recent focus by the U.S. Environmental Protection Agency on bioassessment of lakes, a multimetric index was developed for New Jersey lakes and reservoirs using benthic macroinvertebrates. Benthic samples were collected from reference and impaired lakes with muck and intermediate sediments in central and northern New Jersey during summer 1997. We used a stepwise process to evaluate properties of candidate metrics and selected five for the Lake Macroinvertebrate Integrity Index (LMII): Hilsenhoff Biotic Index (HBI), percent chironomid individuals, percent collector-gatherer taxa, percent oligochaetes/leeches, and number of Diptera taxa. We scored metrics as the fraction of the best expected value (based on all sites) achieved at a site and summed them into the LMII. Evaluation of the LMII showed that it discriminated well between reference and impaired lakes and was strongly related to several potential stressors. Chemical and physical gradients distinguished between reference and impaired lakes, and the LMII summarized these gradients well. The LMII corresponded strongly with land use, but some lakes with more urban land use still achieved high scores. Based on a power analysis, the ability of the LMII to detect differences in condition was sensitive to the number of samples from each lake.