The influence of urbanization on stream insect communities was determined by comparing physical, chemical, and biological characteristics of streams draining 20 catchments with varying levels of urban land-cover in Maine (U.S.A). Percent total impervious surface area (PTIA), which was used to quantify urban land-use, ranged from approximately 1-31% among the study catchments. Taxonomic richness of stream insect communities showed an abrupt decline as PTIA increased above 6%. Streams draining catchments with PTIA < 6% had the highest levels of both total insect and EPT (Ephemeroptera + Plecoptera + Trichoptera) taxonomic richness. These streams contained insect communities with a total richness averaging 33 taxa in fall and 31 taxa in spring; EPT richness ranged from an average of 15 taxa in fall and 13 taxa in spring. In contrast, none of the streams draining catchments with 6-27% PTIA had a total richness > 18 taxa or an EPT richness > 6 taxa. Insect communities in streams with PTIA > 6% were characterized by the absence of pollution-intolerant taxa. The distribution of more pollution-tolerant taxa (e.g. Acerpenna (Ephemeroptera); Paracapnia, Allocapnia (Plecoptera); Optioservus, Stenelmis (Coleoptera); Hydropsyche, Cheumatopsvyche (Trichoptera)), however, showed little relation to PTIA. In contrast to the apparent threshold relationship between PTIA and insect taxonomic richness, both habitat quality and water quality tended to decline as linear functions of PTIA. Our results indicate that, in Maine, an abrupt change in stream insect community structure occurs at a PTIA above a threshold of approximately 6% of total catchment area. The measurement of PTIA may provide a valuable tool for predicting thresholds for adverse effects of urbanization on the health of headwater streams in Maine.