Larvae ofOestrus OvisL. harbored by lambs maintained at a higher temperature had a greater survival rate and matured more rapidly than larvae harbored by lambs maintained at a lower temperature. However, the smaller population of surviving larvae harbored by lambs maintained at a lower temperature produced more mature larvae. Larvae matured more rapidly and in greater quantities in lambs than in yearlings.Although relative humidity had no influence upon the number of adults emerging from pupae, temperature did; 27°C was optimum. Pupae subjected to a temperature below 0°C for brief periods survived, while constant temperatures below 16°C and above 32°C were fatal. A constant temperature of 16°C permitted development of pupae but inhibited emergence of adults.Estimated mortality during the immature stages ofO. ovisin wild populations ranged from 90.2–93.8% for the 1st generation and 98.5–99.1% for the 2nd generation.The incidence of infested sheep during November through February exceeded 90%. The annual mean infestation was 21.9 larvae per sheep. During October and November the sheep harbored a mean of 45 larvae per head.Oestrus Oviswas found to complete 2 generations per year in Kentucky.