Documentation of the seasonal feeding habits and ability to assimilate algae are presented for the first time for a North American grazing stream fish. The chiselmouth, Acrocheilus alutaceus, begins feeding on algal matter in early spring and has deposited large fat reserves prior to an early summer spawning season. A sharp decline in body fat content is then associated with a reduction in food intake at the onset of spawning. A subsequent increase in food intake, possibly with a higher energy value due to the inclusion of animal material, quickly restores fat reserves which then remain high through a period of almost exclusive herbivory (particularly on diatoms) in the late summer and fall. With the onset of winter, the diet of A. alutaceus shifts toward omnivory. Diatoms of the genera Cymbella, Gomphoneis, Synedra, and Melosira form the bulk of the diet. Both diatom and green algal food sources were assimilated by A. alutaceus, but at relatively low efficiency. The efficiency of assimilation of Ulothrix varied inversely with ration size, while the assimilation of diatoms was unaffected by ration size. Fiber was not assimilated. The protein fraction was assimilated more efficiently (44%) than total energy (33%). The order of preference evidenced in A. alutaceus feeding trials was: diatom > bluegreen > green > "mix" > moss. The ratio of digestible protein to digestible energy (P/E) in the food sources proved an excellent descriptor of observed preference. However, a linear combination of fiber content (as a negative factor) and gross P/E (as a positive factor) provided an analytically simpler descriptor and also predicted the correct order of preference. Chiselmouths apparently live much longer (22+ yrs) than was previously known (6 yrs). Formation of the opaque zone, generally recognized as indicating a growth period, is most evident on A. alutaceus otoliths from July through October. This coincides with the period of highest food intake and peak contribution of diatoms to the natural diet. / Graduation date: 1991