Electron microscopy of the dermis of the 9-banded armadillo, Dasypus novemcinctus, reveals an intimate relationship of mast cells to peripheral nerves. Mast cells are routinely found in the dermis in close proximity to nerves, and mast cells are also present within the perineurial cell sheath in the endoneurial space proper. These cells contain electron opaque granules and exhibit numerous surface folds as is characteristic of other species. The degranulation of the mast cells by injection of 0.5% aqueous trypan blue reveals sequential exocytosis of granules, similar to that described for the rat. Degranulation occurs in most dermal and endoneurial mast cells following a single intradermal injection of trypan blue. A small number of mast cells partially degranulate and a few exhibit no degranulation, suggesting a heterogeneous population. It is also noted that mitochondrial morphology differs in degranulating cells, in that they become swollen and vacuolated. The results of this study are discussed in light of previous results reported for other species, mainly at the light microscope level.