Scabies due to infestation by Psoroptes oris was monitored from 1979 through 1981 in Nelson's desert bighorn sheep in northwestern Arizona. Visual examinations for ear lesions and evaluations of ear scrapings were used to assess the prevalence of mites in sheep that were live-captured or killed during the hunting season. The prevalence of psoroptic scabies in live-captured sheep for the years 1979, 1980, and 1981, was 25%, 25% and 0% respectively, and was higher in rams (1979, 65% 1980, 100%) than in ewes (1979, 11% 1980, 0%). The severity of ear lesions was also greater in the males. The prevalence in harvested rams was 23% 44% and 7% respectively, for the years 1979–1981. The severity of the ear lesions was highest in 1980. Lesions were confined to the ears. An aerial census of bighorns in the Black Mountains during May of 1981 showed a 38% decrease in total observations from the previous year and a drop in the ewe lamb ratio from 0.52 in 1980 to 0.29 (P ≤ 0.01) in 1981. In the Lake Mead area, there was a population census decline of 33% in 1981 and a non-significant decline in the ewe lamb ratio from 0.46 to 0.40. The population decline in 1981 and the increased prevalence and severity of psoroptic scabies in the 1980 survey in both areas were believed to be related to the high bighorn population densities in 1980 and the prevailing drought.