The distribution of carbon and nitrogen stable isotope ratios in macroalgae in Hiroshima Bay was examined seasonally from June, 1999 to June, 2000. A total of 52 species including 109 samples were collected from the eutrophic northern area and the mesotrophic central area of the bay. The macroalgae were mainly distributed from -23.0 to -11.0‰ in δ¹³C and from 7.0 to 12.0‰ in δ¹⁵N. Both the macroalgal δ¹³C and δ¹⁵N were significantly more enriched in the northern area than in the central area with intraspecific local differences of 3.5±2.6‰ (mean±S. D. ) in δ¹³C and 1.8±0.8‰ in δ¹⁵N, and thus the distribution on the δ¹³C-δ¹⁵N map differed between these areas. It was therefore suggested that it would be possible to identify the feeding area of herbivorous animals grazing on a specific macroalgal species on the basis of the carbon and nitrogen stable isotope distribution. The macroalgal δ¹³C and δ¹⁵N also varied seasonally with an intraspecific maximum variation of 5.9‰ in δ¹³C and 4.0‰ in δ¹⁵N, Particularly the δ¹⁵N in the macroalgae collected from the central area increased in winter. The δ¹³C of particulate organic matter in the surface layer in the central bay averaged -20.1±1.7‰, ranging from -22.3 to -16.9‰, and overlapped with the macroalgal isotope distribution. We must pay attention to this isotopic overlap between POM and macroalgae for identifying carbon sources for local animals using analysis of stable isotopes.