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Circadian rhythms of activity were compared in ground-dwelling Tenebrionid beetles from the Kara Kum sand desert: Trigonoscelis gigas Reitter, Trigonoscelis sublaevicollis Reitter, Pisterotarsa gigantea Fish.-W, Sternodes caspicus Pall, Blaps faustii Seidlitz, and Ocnera imbricata Fish.-W For the observations, artificial pens (enclosures) 60 x 60 cm and 80 cm deep were arranged in field conditions and filled with various kinds of sand to simulate natural habitats as closely as possible. The activity of individually marked beetles was assessed visually and recorded every 30 min. The beetle species studied had different types of behavior, from strictly crepuscular to strictly nocturnal. Closely related species sharing one habitat differed in activity pattern-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. In experiments with T gigas, we failed to stimulate individual variability among beetles in circadian pattern by applying additional light and heat at night. Therefore, the circadian rhythms in field conditions were found to be remarkably stable, which can be explained by our model of the beetle circadian system consisting of two strongly coupled oscillators. However, in laboratory conditions, we previously observed a wide range of individual variability in free-running circadian rhythms. The strong coupling between two putative endogenous circadian oscillators is a crucial tool for beetles to survive in a harsh arid environment. Such organization of the circadian system ensures synchronous activity of beetles despite individual differences, thus preserving the variability of circadian clock properties in a population, which increases chances for survival of a population in a changing environment.