ABSTRACT. The horn fly (Haematobia irritans) is particularly unique among the parasites that primarily affect Brazilian cattle farming. Appropriate control strategies fundamentally depend on epidemiological knowledge, which is particularly scarce in Northeastern Brazil. This study aimed to elucidate the ecology of the immature horn fly in the semiarid region of Brazil. Bovine fecal masses were collected and covered with emergence traps to collect the horn fly for sexing and counting. Weather records of the region were concurrently acquired. A total of 11,390H. irritans were collected from 601 fecal masses, with a sex ratio of 0.9:1 (male:female). Horn fly emergence was observed in 78% (15%?100%) of the fecal masses, varying from 1 to 185 (mean = 23.3) flies/fecal mass, predominantly at the beginning of the rainy season. The minimum period for egg-to-adult development of H. irritans varied from 7 to 11 days, indicating the occurrence of 30 generations per year in the region. Rapid development of the immature horn fly in the semiarid region, throughout the year may results in a high number of generations and infestations in cattle herds. © 2020