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Circadian rhythms of locomotor activity of the desert beetles T. gigas were entrained with skeleton photoperiods (2x2 hr per circadian cycle 30 Ix green LED light pulses). The Zeitgeber period was stepwise reduced by 1 hr down to 22 hr or increased up to 26 hr. Within the range of entrainment, the phase angle Psi of a circadian rhythm with respect to light depends upon the period of Zeitgeber differently for the morning (M) and evening (E) peak: M is easier to advance, while E is easier to delay. Beyond the range of entrainment both peaks became free-running with some relative coordination. Masking (direct stimulation of activity by light) occurred only during the subjective night, and never in subjective day. In few cases one of two peaks became free-running while its counterpart remained entrained, suggesting that each of the two peaks has its own visual input and can be entrained by light. These results are in agreement with the difference in the PRC shape for the M and E peaks, and support the hypothesis that M and E peaks are controlled by two functionally separate oscillators that have polar different properties, and are extremely strongly mutually coupled with phases locked at about 180 degrees.