The autotrophic ammonium-oxidizing bacterium Nitrosococcus oceanus was studied in the brackish lake of Ganzirri by cultural and immunofluorescent methods. The preparation of the specific antiserum in rabbits is reported. The polyclonal antiserum for Nitrosococcus oceanus cross-reacted with other ammonia oxidizing strains, but not with other marine bacteria. Temporal changes were determined by taking monthly water samples from a station 6 m deep and the distribution in sediments was investigated in four stations. Isolation of a strain of Nitrosococcus sp. was obtained from a sediment sample collected in December. The abundance of Nitrosococcus spp. bacteria correlated positively with particulate organic carbon (POC), particulate organic nitrogen (PON), temperature and total bacteria, whereas there was a negative relationship with oxygen tension. No correlation was found between immunofluorescent and MPN counts of Nitrosococcus spp. bacteria.