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Biting insects impose Costs on hosts, including decreased feeding or resting time as the result of disturbance, blood loss, and disease transmission. Insect-repelling behaviors, Such as ear-flicking, head-shaking, stamping, and grouping, have evolved in many ungulate species to minimize these costs. We studied female desert bighorn sheep (Ovis canadensis mexicana) at Red Rock Wildlife Area, New Mexico, during the summers of 1999 and 2000. We tested the predictions that: 1) bighorn sheep will increase insect-defense behavior when biting insects are more abundant, and 2) close aggregation of sheep will decrease the per capita insect harassment by means of a dilution effect. Numbers of midges and other biting insects increased in association with rising temperature and decreased with increasing wind speed. Ewes performed between 0 and 78 car-flicks/min, and >5,000 ear-flicks over the course of a 12-h day. As predicted, the rate of ear-flicking was positively correlated with counts of biting insects, indicating that car-flicking was a direct response to the irritation of attacking insects. We also found a negative correlation between the number of sheep clustered together within 1 body length and ear-flicking rate, suggesting that insect harassment is diluted when bighorn sheep bunch together. Bighorn sheep generally bedded on tipper slopes and rocky outcrops exposed to gusts of wind. These results indicate that ear-flicking, grouping, and microhabitat choice might be important strategies for reducing the costs of biting insects in desert bighorn sheep.