The natural particulate food of larval Coquillettidia perturbans was studied through gut analysis using staining with 4'6-diamidino-2-phenylindole and epifluorescence microscopy. Bacteria (cocci, rods, spirochetes, purple bacteria and cyanobacteria), detritus, euglenoid protozoans and algae (desmids and diatoms) comprised the majority of particulate food, in order of abundance; other protozoans were rare, and hyphal forms (actinomycetes and fungi) were not observed. Abundance of food items in guts did not vary greatly among sampling months (June-December, 1987) or sampling sites. Fourth instars had a greater proportion of euglenoids and algae in their food than did younger instars. Observation of larval feeding behavior showed that fourth instars oriented upside down and fed mainly by suspension feeding, with occasional brushing from sediments and root surfaces. Microtrichia on filaments of the lateral palatal brushes of fourth instars indicated that the larvae were adapted to collect fine particles.