By the method of non-invasive on-line recording and processing of photoplethysmograms of testaceous invertebrates, the circadian rhythm of cardiac activity was studied in the crayfish Pontastacus leptodactylus by recording for several months the heart rate (HR) and stress-index (characteristics of variational pulsometry). The crayfish were kept in the natural running water in the regime of natural illumination alteration (the first group) or at constant artificial illumination of low intensity (the second group). The circadian rhythm was more frequent and more distinct in crayfish of the first group. The criteria were established to determine the appearance and stabilization of the nocturnal, active rhythm phase: an increase of HR by more than 30% as compared with daytime rest period and duration of such increase for at least 2.5 h. The stress-index has been shown to be a reliable parameter of the beginning of the nocturnal phase of cardioactivity, while preservation of the typical circadian rhythm can be considered as a bioindicator in the biomonitoring systems of the quality of superficial waters.