The aim of the study has been to estimate the effect of culture conditions and a culture site on magnesium (Mg) concentrations in freshwater fish. The study encompassed tissues (blood) and organs (gills, liver, kidney, dorsal muscles) of five fish species: common carp (Cyprinus carpio L.), rainbow trout (Oncorhynchus mykiss Walbaum), Siberian sturgeon (Acipenser baeri Brandt), northern pike (Esox lucius L.) and grass carp (Ctenopharyngodon idella Valenciennes). A total of 125 fish comprised 25 individuals of each species, aged 6, 12, 18 and 24 months. The fish were cultured in privately owned fish breeding ponds (Western Pomerania, Poland). Tissue and organ samples were wet mineralised in concentrated HNO3 in a CEM MDS 2000 microwave oven. Magnesium concentrations were determined by inductively coupled plasma atomic emission spectrometry (ICP-MS) in a Jobin Yvon type JY-24 apparatus. The research had an approval of the Polish Local Ethics Committee no 9/05. The magnesium concentration in the tissues and organs ranged from 26.3÷174.2 mg kg–1 w.w. The lowest Mg concentration was found in the gills of rainbow trout (26.3±5.4 mg kg–1 w.w.), and the highest – in the liver of rainbow trout (174.2±27.6 mg kg–1 w.w.). The magnesium concentrations were also significantly affected by the type of feed.