The present study is an electron microscopic analysis of the numbers and sizes of axons in the connectives and nerves of the medicinal leech. In either the right or left connectives for ganglia 14-18, there are approximately 2860 (+/- 294 S.D.) axons. Ninety-seven percent of these axons are less than one micron in diameter. The median connective, Faivre's nerve, contains 97 (+/- 3 S.D.) axons, and 94% of these fibers are smaller than one micron. In the peripheral nerve roots for ganglia 14-18, there are approximately 2351 (+/- 311 S.D.) axons. Ninety-eight percent of the axons in the nerves are less than one micron in diameter. Thus, there are approximately 20,000 axons associated with each of the segmental ganglia 14-18 in the leech, and the vast majority of these fibers are less than one micron in diameter.