Determined the statistical properties of the index of biotic integrity (IBI) from electrofishing samples collected from Ohio/streams. Although IBI is widely used to evaluate the condition of water resources by biologists and resource managers, expanding its role as a regulatory tool depends on statistical validation of its precision and power. The authors constructed an additive variance model for IBI and tested the assumptions of that model directly with a bootstrap resampling algorithm and using field data. IBI can distinguish between five and six nonoverlapping categories of biotic integrity based on a model that includes the effects of measurement error, variability of fish assemblages through time, and statistical interaction of location and time. IBI is thus an effective monitoring tool that can be used to communicate qualitative assessments to the public and policy makers or to provide quantitative assessments for a legal or regulatory context based on confidence intervals or hypothesis testing procedures.