ABSTRACT. A qualitative electron microscopical examination of perltoneal phagocytes in winter flounder Pleuronectes americanus after brief (i.e. 30 or 60 min) exposure to formalin-killed Bacillus cereus or Yersinia ruckerj revealed that both neutrophils and macrophages had phagocytozed bacteria. Neutrophils obtained from both the perltoneal cavity and from the head kidney were identified on the basis of their overall cytological appearance and more specifically on the presence of elongated, striated (fibrillar) appearing granules in their cytoplasm which stained positively for peroxidase. Macrophages recovered from the peritoneal cavity lvere phagocytic like neutrophils; however, they were not observed to contain peroxidase. The surface membrane changes observed for 'activated' neutrophils and macrophages which had engulfed bacteria were consistent \\rith those reported for other fish species and higher vertebrates. Images of aggregated cells suggestive of cellular communication between lymphocytes, macrophages, and/or neutrophils are provided.