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Circadian rhythms of activity were compared in ground-dwelling Tenebrionid beetles from the Kara-Kum desert: Trigonoscelis gigas Reitter, T. sublaevicollis Reitter, Pisterotarsa gigantea Fish.-W., Sternodes caspicus Pall, Blaps faustuu Seidlitz, and Ocnera imbricata Fish.-W. Imagoes of these Tenebrionids can be encountered on the ground surface from early spring and throughout summer until late autumn. The beetle species studied had different activity patterns, from strictly crepuscular (T. gigas and S. caspicus) to strictly nocturnal (T. sublaevicollis and O. imbricata). P. gigantea and B. faustii were more active in morning and evening hours. The circadian rhythm protects beetles from being active in the daytime independently of weather or season. The beetle remained deep in sand during their rest time. The closely related species T. gigas and T. sublaevicollis, sharing the same habitat, differed in the pattern of activity: they occupied different time niches. Hence, their circadian rhythms contribute to reproductive isolation. In contrast, non-closely related species could have similar circadian patterns as a result of convergent evolution. The circadian rhythm in Tenebrionid beetles is considered to be a powerful tool for adaptation to the conditions of the Kara-Kum desert. The rhythm synchronizes the activity of beetle at favorable hours, thus allowing both species to meet for mating and ensuring that no beetle will appear on the surface during unfavorable hours of the daytime.