Larval feeding rates ofSimulium tescorumStone and Boreham,S. vittatumZetterstedt, andS. argusWilliston in streamlets and that ofS. vittatumin the Colorado River were studied by releasing a water suspension of a fluorescent water disperable particulate dye. The dye particles were readily ingested, forming a discrete plug in the gut visible to the naked eye. The displacement of the dye plug posteriorly provided an easy assessment of the feeding rate in these filter feeding larvae.Feeding rate in younger larvae was greater than in older larvae. Younger larvae ofS. tescorumvoided the dye plug in anywhere from 10 min (water temp 86°F) to 35 min (water temp 55°F), while the older larvae (4–6th instars) voided the dye plug in 20–30 min at the high temperature and 55 min at the lower temperature. Larvae ofS. vittatumandS. argusvoided the plug more slowly than the larvae ofS. tescorum.Larvae ofS. tescorumandS. vittatumshowed no rhythmic feeding activity. They fed at essentially the same rates at all hours of the day.TheS. vittatumlarval feeding rate in the Colorado River was essentially the same as that found in small streamlets.