Twenty-two stations on the Pecatonica River and tributaries were sampled for water and sediment quality, macroinvertebrates, fish and habitat from spring 1984 through winter 1985. From these data, environmental quality was evaluated utilizing various indices including water quality (WQI), macroinvertebrates (MBI), fish (AIBI) and habitat (PIBI). Individual stations were rated for degree of support of aquatic life and classified according to the Biological Stream Classification (BSC) system used for Illinois streams. Mean index values indicated fair to good environmental conditions providing full aquatic life use at 13 of 22 stations (59%), partial/minor support at 7 of 22 stations (32%) and partial/moderate support at 2 of 22 stations (9%). Mean index values were 33.8 (fair to good) for water, 4.9 (fair to good) for macroinvertebrates, 38 (fair to good) for fish and 38 (fair to good) for habitat. According to the Biological Stream Classification system, 68.2% of the stations were classified as C (moderate aquatic resource), 22.7% as B (highly valued aquatic resource), 4.5% as A (unique aquatic resource), 4.5% as D (limited aquatic resource) and none as E (restricted aquatic resource).