The accessory cells of rat, guinea-pig and chick sympathetic ganglia have been studied in tissue culture with phase-contrast microscopy, time-lapse cinematography and fluorescence histochemistry. Small intensely fluorescent (SIF) cells have been described for the first time in culture. Satellite cells, which are closely associated with nerve cell bodies, and interstitial cells, which are dispersed throughout the ganglion, appeared to be identical cell types; they do not correspond to oligodendrocytes of the central nervous system as has been suggested previously. However, a small number of cells closely resembling central oligodendrocytes were present. Astrocyte-like cells have also been seen occasionally, although their identity is not clear. Several other cell types were present and have been identified as Schwann cells, sheath cells, fibroblasts, perineural epithelium, macrophages and endothelial cells.