The Mauthner cells (M-cells) are a pair of neurons found in the medulla oblongata of most fish and amphibians. This neuron is not the same size in all teleosts. Based on the hypothesis that differences in M-cell size might be related to fish family and possibly fish habitat, M-cell and nuclear size were compared between fish families. In order to minimize effects of fish length on cell or nuclear size, statistical treatment of the data was made for four separate length classes. The results indicate at least three groupings by cell or nuclear size. The cell is large in fish from the Salmonidae, Catostomidae and Cyprinidae, small in the Stichaeidae, Cottidae and Pleuronectidae and absent in Batrachoididae, Lophiidae, Ogcocephalidae and Cylopteridae. In general a correlation exists between fish that lack or have small M-cells and a demersal habitat. Since these fish may rely on camouflage for protection against predation, the M-cells may have altered excitability or function as compared to more active fish in which the M-cell is known to function in initiating the startle response (larval zebrafish and goldfish). Differences in input to M-cells of different sizes in conjunction with known electrophysiological properties of the goldfish M-cell are discussed in relation to the function of this neuron.