Measurements of delta C-13, delta N-15, and C/N on diatom-bound organic matter were made over the Holocene and Last Glacial Maximum (LGM) from three sediment cores in the Southern Ocean, one each from the Atlantic, Indian, and Pacific sectors. The site in the Scotia Sea (Atlantic sector) differed considerably from the other two sites by having markedly lower delta C-13, more variable delta N-15 and C/N ratios, and a sedimentary diatom assemblage that was never dominated by Fragilariopsis kerguelensis. Although environmental parameters certainly have a strong impact on the isotope ratios, delta C-13 is also correlated to the proportion of F. kerguelensis in the three cores investigated here (r(2) = 0.8). Extreme values of delta C-13, delta N-15, and C/N at the Last Glacial Maximum were also related to the abundance of winter stages of Eucampia antarctica. These results suggest that diatom specific isotope records should be interpreted in conjunction with information on the species composition of the samples.