The ultrastructural localization of glucagon in the presence of Scyliorhinus canicula was investigated. We used a post-embedding immunoelectron microscopy method on pancreatic samples fixed in glutaraldehyde and osmicated before embedding. Contrasting with uranyl acetate and lead citrate was also performed after immunolabelling, but best results were obtained with uranyl acetate only. Glucagon-like immunoreactivity was located in round granules (300-600 nm) surrounded by a limiting membrane. The matrix varied in electron density and exhibited a dense core surrounded by a less dense mantle. The granules were seen in two different cell types, which differed in the electron density of their cytoplasm. Glucagon-immunoreactive cells were the largest pancreatic cells types and were often localized near somatostatin-containing cells.