The effects of current velocity, food particle density and water temperature on the feeding rate of ultimate instars of Simulium vittatum Zetterstedt were studied in the laboratory utilizing an aerated jar rearing system and a particulate fluorescent dye. Feeding rates increased with increase in current up to a point, but a curvilinear relationship between current velocity and feeding rate was observed over the whole range. After 50 min of feeding on dye particles at 26 and 39 cm/sec (19C, 75 ppm) the dye filled 42 and 52% of the gut respectively. When the velocity was raised to 53 cm/sec only 30% of the gut was filled. When a wide range of particulate densities was employed, a significant difference in feeding rate was observed between larvae feeding at high and low particulate concentrations (19C, 26 cm/sec). The dye in larvae feeding at 1 and 10 ppm filled only 31 and 32% of the gut respectively. At concentrations between 50 and 100 ppm the average filling was 44% of the gut. Temperature of the medium affected feeding rates in larvae exposed to dye particulates (73 ppm, 26 cm/sec) at 4, 10, 15, 19, 24 and 28 C. The feeding rates at 4 and 10 C were significantly lower than those at 15-28C. There was no evidence that peristalsis aids in the posterior movement of the food column in developing larvae.