The wide range of functional activities of circulating and sessile insect hemocytes expresses itself in highly specialized cytological terms. Electron microscopic studies carried out in five species of normal and experimentally manipulated cockroaches, in conjunction with light microscopic information, reveal a broad spectrum of structural organization and an apparent capacity for cellular modulation. In addition to conventional organelles, these hemocytes contain a class of unusual cytoplasmic inclusion bodies which seem to undergo striking transformations in response to specific functional demands. A variety of transitional forms attests to the existence of close links between tubule containing (type 1), electron dense (type 2), and large globular (type 3) inclusion bodies, and reveals the derivation of yet another special (lamellated, fusiform) inclusion from type 2 bodies. Confluence of the type 3 configurations into still larger lacunae may precede the release of their contents into the hemolymph, a process whose major effect seems to be the initiation of the clotting process.