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Running title: Circadian rhythms in cave-dwelling fish Astyanax Abstract:Circadian regulation has a profound adaptive meaning in timing the best performance of biological functions in a cyclic niche. However, in cave-dwelling animals (troglobiotic) a lack of photic cyclic environment may represent a disadvantage for persistence of circadian rhythms. There are different populations of cave-dwelling fish Astyanax mexicanus in caves of the Sierra El Abra, Mexico, with different evolutive history. In the present work we report that fish collected from El Sótano la Tinaja show circadian rhythms of swimming activity in laboratory conditions. Rhythms observed in some of the organisms entrain to either continuous light-dark cycles or discrete skeleton photoperiods tested. Our results indicate that circadian rhythm of swimming activity and their ability to entrain in discrete and continuous photoperiods persist in some organisms that might represent one of the oldest populations of cave-dwelling Astyanax mexicanus in the Sierra