The mitochondrial ultrastructure in ventricle cardiomyocytes of three gastropod molluscs (Clione limacina, Helix pomatia, Lymnaea stagnalis) has been studied. Mitochondria in cardiomyocytes of these molluscs are connected by intermitochondrial contacts of the same morphology as intermitochondrial contacts in vertebrate cardiomyocytes. Their numbers in cardiomyocytes of the above molluscs being, respectively, 61, 35.1 and 29.2 contacts per 100 mitochondria. In Clione limacina cardiomyocyte contractile elements located on the periphery of cell occupy 21.1% of the cytoplasm volume. Mitochondria form a core making large dense central accumulations taking up 54.9% of the cytoplasm volume. Numerous mitochondria have vesicular or tubular cristae and light matrix. Unlike cardiomyocytes of Clione limacina, in Helix pomatia and Lymnaea stagnalis contractile material predominates in cardiomyocytes occupying 43.7% and 49.2% of the cytoplasm volume, respectively. Mitochondria located on the periphery and in the center of cardiomyocytes in Lymnaea stagnalis and Helix pomatia occupy 31 and 32.5% of the cytoplasma volume, respectively. Mitochondria in cardiomyocytes of both these molluscs have plastic cristae and dense matrix. The differences in cardiomyocyte mitochondriom organization in the studied molluscs can be explained by different functional heart loading in these due to different levels of their locomotor activity.