Sixty-two years since last observed alive, Mormotomyia hirsuta Austen, the “terrible hairy fly”, was found inside and outside a large, cave-like cleft boulder at the summit of Ukasi Hill in eastern Kenya, the type locality of the species. Adults were observed climbing the walls of the boulder and walking on thick layers of bat guano, in which larvae and puparia were also discovered. Large numbers of M. hirsuta were observed on and at the base of the northern side of the boulder, which at the time of capture experienced continuous shade during daylight hours. Only three individuals were observed at the southern opening, exposed to direct sunlight and hot, dry conditions. A collection of vertebrate bones and skulls from layers of guano both inside and outside the cleft revealed several vertebrate associates, including two species of Chiroptera, Chaerephon cf. bivittatus (Heuglin) and Tadarida aegyptiaca (E. Geoffroy), which are probably the two major guano-producing species responsible for the larval breeding medium. Male-biased sexual size dimorphism was pronounced in adult M. hirsuta, with seven body-part measurements, including legs, larger by 33–61 % in males than females. Males demonstrated isometric growth while female growth was allometric. In contrast to males, female head and thorax lengths did not increase proportionally with leg length. Estimates of genetic diversity in the Ukasi population show higher than expected allelic diversity and indicate possible gene flow and frequent population bottlenecks. To promote the conservation of this endangered species, a joint effort has been initiated between the International Centre of Insect Physiology and Ecology, Nairobi and the National Museums of Kenya, Nairobi, to gazette the Ukasi hill area as a protected site.