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In the Karakum Desert, Turkmenistan, regardless of season and weather, the beetles Trigonoscelis gigas are only active in the morning and evening, while T sublaevicollis are strictly nocturnal. In the laboratory, we studied the daily rhythm of activity of T gigas and T sublaevicollis according to the following pattern: 5 days under a light-dark cycle of 15 : 9 hours (LD 15 : 9), then 10 days in constant darkness (DD), and then another 10 days under alternating 1-hour pulses of light and darkness (LD 1 : 1). In all modes, the temperature was 25 degrees C. At LD 15 : 9, beetles of both species maintained a 24-hour period and a natural pattern of the activity rhythm. In DD, the circadian rhythm was observed to run with a period equal to 23.5 +/- 0.3 h (n = 40) in T gigas, versus 23.6 +/- 0.4 h (n = 40) in T sublaevicollis. Under DD conditions, in T gigas the morning and evening peaks of activity merged, resulting in a rhythm with only one peak. Under LD 1 : 1, both T gigas and T sublaevicollis recovered a 24-hour period of the rhythm, wherease the rhythm of T gigas regained a two peak mode. Hence, our research confirmed W.B. Tshernyshev’s (1980) assumption of a 24-hour period of a free-running endogenous rhythm and of a deforming impact of constant conditions on the rhythm.