Aggregation sites for male Cuterebra grisea Coquillett were found near areas where deermice were infested by botfly larvae. At these sites which were near streams or roadsides, male C. grisea flew over heat-reflecting surfaces which often contained objects taller than their surroundings. Each male chased intruders, including other males, from a territory over which it flew at regular intervals in an oval or figure-of-eight pattern. They flew in mornings during July and August provided the shade temperature was above 19.5°–20 °C and the sun was shining or had been shining. On very warm mornings when 20 °C was reached early in the morning, males did not fly until about 33/4–4 hours after sunrise. Flight almost always ended by midday. Female flies were rarely seen because they appeared only long enough to be mated. The data on C. grisea led to the discovery of similar male aggregation behavior of the pack rat botfly, C. tenebrosa Coquillett.