A field and laboratory investigation of the food and feeding behavior of larvae of the potamanthid mayfly Anthopotamus verticis (Say) was conducted from 1989 to 1991 on a population from the Tippecanoe River, Indiana (USA). Gut content analyses indicated that all size classes of larvae are detritivores, with over 95 % of food consisting of fine detrital particles. Videomacroscopy indicated that all size classes of larvae are filter feeders, able to utilize both active deposit filter feeding and passive seston filter feeding cycles in their interstitial microhabitat. Deposit filter feeding initially incorporates the removal of loosely deposited detritus with the forelegs. Seston filter feeding initially incorporates filtering by long setae on the forelegs and palps. Mandibular tusks are used to help remove detritus from the foreleg setae. A SEM examination of filtering setae indicated they are hairlike and bipectinate, being equipped with lateral rows of setules. The results show that previous assumptions that potamanthid larvae were collector/gatherers are erroneous. The results are applicable to congeners, and all potamanthids, including Palearctic and Oriental elements, are hypothesized to be filter feeders, differing only in some details of behavior.