This study was aimed at developing the nucleolar biomarker and the micronucleus test on in vivo fish fin cells for assessing water cytotoxicity and genotoxicity. Both biomarkers can be used either jointly or separately on fins of the same fish during the experiment. For studying the nucleolar characteristics, small pieces of the fin edge were cut several times during 30-180 min of fish exposure. For micronucleus testing, the fin tissue regenerating after its cutting was investigated after 2-5 days of fish incubation. Effects of copper (0.1 and 2.5 mg/L), cadmium (0.005 and 1.0 mg/L) ions and chloral hydrate (400 and 800 mg/L) solutions were studied on cells of common carp (Cyprinus carpio L.), crucian carp (Carassius auratus gibelio Bloch.), and Mozambique tilapia (Tilapia (Sautherodon) mossambica) using a set of nucleolar characteristics (the number of nucleoli per cell, the size of a single nucleolus, and the percentage of cells with heteromorphic paired nucleoli) and the frequencies of cells with micronuclei and double nuclei. Substantial changes in parameters of nucleolar activity of fin cells were found to be caused by cadmium and copper impact. In comparison to blood cells, gill and fin cells were more sensitive as demonstrated by their nuclear damages after the chloral hydrate influence. Fin cells were useful to determine periodically cytotoxic and genotoxic effects of organic and inorganic substances in the same individual fish without any disruption of its physiological functions.