1. 1. We investigated the electrophysiological properties of 18 identifiable neurons in three leeches: Haemopis marmorata, Hirudo medicinalis and Macrobdella decora. 2. 2. The paired serotonergic Retzius cells are indistinguishable among the species in terms of somata positions, branching patterns, electrophysiological parameters and muco-effector functions. 3. 3. The T, P and N mechanosensory cells in these three species are virtually identical by virtue of their sizes, locations, axonal branching, electrophysiological properties, responses to dermal stimulation and receptive fields. 4. 4. The L motor neurons of all the leeches are electrotonically coupled, have equivalent cell positions and axonal branches, and control the longitudinal muscles. 5. 5. We have investigated several properties of 12 neurons in Hirudo and Haemopis and found no differences between their S interneuron, heart excitor motor neurons and their monoamine-containing neurons. 6. 6. We found no differences between 30 homologous neurons in the C.N.S. of these leeches at any level of organization: membrane properties, central connectivity, morphology and axonal branching, peripheral connectivity or functional roles. The extremely conservative stereotypy of these cells is impressive. © 1977.