Fish are important biotic controls in many stream systems, and may be crucial in arid land streams. This study aimed to assess the importance of fish in Southwestern streams by investigating the function of two sucker species, Catostomus insignis and Catostomus clarki, in Bonita Creek, a low elevation tributary of the Gila River in southeast Arizona.  C. clarki and C. insignis are closely related species that often coexist but differ in feeding modes and microhabitat use, the former an herbivore and the latter an omnivore.  These species appear to be dominant members of the stream community, strongly influencing standing stocks at lower trophic levels. Small-scale (0.09 m²) exclosure experiments suggest that grazing suckers influence periphyton abundance, quality, and distribution. In treatments where fish were excluded, large quantities of flocculent organic matter accrued on benthic substrates and effectively inhibited surface growth.  In contrast, flocculent material was typically absent in treatments accessible to fish, and a firmly attached 1-2 mm layer of periphyton was present.  Grazing fish reduced algal standing crop (chlorophyll a and ash-free dry mass), but increased the proportion of autotrophs in organic matter and the amount of organic matter per unit mass, increasing food quality.