In Antarctic krill, Euphausia superba, sampled by a Japanese scientific observer onboard a krill fishing vessel in the winter of 2003 and 2006 in the South Georgia region, the Antarctic Ocean, approximately 2-5% of sub-samples of 100 krill bore small black spots. The black spots were most often found on the cephalothorax of the body. Three bacteria were isolated from these black spots, and classified into either Psychrobacter or Pseudoalteromonas by the sequences of 16S rRNA genes. Histological observations revealed that the black spots were melanized nodules. A single melanized nodule often contained more than one type of morphologically distinct bacterial cell. More than three bacterial species or strains were also confirmed by in situ hybridization for 16S rRNA. The melanized nodules were almost always accompanied by a tumor-like mass of unknown large heteromorphic cells, which seemed to be derived from a gonadal tissue. These results suggest that the krill were affected by bacterial infections, whereas the presence of multiple bacterial species suggests that the infections were likely to be secondary. The development of the tumor-like cell mass in the gonad may be the primary condition, although this requires further detailed study.