SummaryThe efficacy of censusing black rhinoceros (Diceros bicornisLinnaeus) populations from the air was tested. Repeated aerial counts were made of the black rhinoceros population centered on Qlduvai Gorge, Tanzania, where the number of rhinoceros on the ground is known accurately. Even under the most ideal conditions only 50% of the population was detected by observers in an aircraft, and factors accounting for this are considered. It is concluded that the light aircraft is of limited value in providing estimates of black rhinoceros populations, and estimates based purely on aerial counts are subject to considerable variation.