1. The cellular organization of the ninth and tenth paravertebral sympathetic ganglia in the bullfrog was studied with intracellular and extracellular recording methods. An isolated preparation was used in which anatomical details of individual cells could be resolved while making physiological measurements. This permitted the characterization of neurones in terms of their size, the segmental origin of their cholinergic innervation, and their orthodromic and antidromic conduction velocities. With these criteria, three classes of sympathetic neurones were identified. 2. As in previous studies, C cells were distinguished from B cells by the origin of their innervation. C cells are innervated by slowly conducting axons (0.4 m/sec) from spinal nerves 7 and 8 and B cells are innervated by rapidly conducting axons (2.4 m/sec) from the sympathetic chain above ganglion 7. 3. In earlier work it has been suggested that the conduction velocity of a preganglionic axon generally matches that of its target neurone. In this study we have characterized a large group of B cells for which this is not true. The axons of B cells fall into a rapidly conducting group (2.0 m/sec) and a slowly conducting group (0.6 m/sec). In contrast, C neurones, like their preganglionic inputs, have only slowly conducting axons (0.3 m/sec). Consequently, neurones have been classified as C type, fast B type, and slow B type. Fifty-nine percent of the B cells that we studied were slow B cells. These findings were corroborated by measurements of compound extracellular responses in post-ganglionic nerves. 4. Some neurones can be identified also by the size of their cell bodies. C cells are about 30 microns in diameter while B cells are about 50 microns in diameter. In our sample, 96% of the cells with radius less than 16 microns were C cells and 94% of the cells with radius greater than 21 microns were B cells. However, fast B cells could not be distinguished from slow B cells by size.