<p>This study investigated ontogenetic growth changes in total mercury (THg) concentrations, δ¹³C, δ¹⁵N, and δ¹⁸O values of harbour porpoises stranded along the coast of Hokkaido, Japan, focusing on differences between males and females. Body length (BL) of the studied harbour porpoises was greater in females than in males. THg preferentially accumulated in the livers rather than in the muscles. The THg concentrations in male and female livers increased sharply and moderately when the BLs exceeded approximately 1.5&#xa0;m and 1.6&#xa0;m, respectively, at which BL the harbour porpoises reached sexual maturity. Significant positive correlations were found between the BL and the log Hg concentrations in the livers and muscles of both males and females. δ¹⁵N levels in muscles were higher in calves than in weaned porpoises, likely due to the consumption of ¹⁵N-enriched milk. The δ¹³C and δ¹⁵N values in immature males and females varied considerably, probably due to the variety of prey species at low THg concentrations. The δ¹³C and δ¹⁵N values in mature males and females increased with increasing BL, suggesting an increase in the trophic level of preys and a possible association with increases in the THg concentrations in their muscles and livers. A significant positive correlation was found between the BLs and the&#xa0;δ¹⁸O values in the muscles from both males and females. A significant correlation was also found between the log THg concentrations and the δ¹⁸O values but not with the δ¹³C values and δ¹⁵N values. These correlations suggest feeding shifts towards deep benthic waters with growth.</p>