This study deals with the morphology and ultrastructure of the mesonephros in adult caecilians of the species Geotrypetes seraphini. Based on serial sections in paraffin and araldite, nephrons are reconstructed and the cellular characteristics of different nephron segments described. The long and slender mesonephric kidneys of G. seraphini are broadest caudally and taper toward the front, where the organs are divided into smaller segmental divisions. Two nephron types can be distinguished on the basis of their connections to the coelom and their position within the nephric tissue: ventral nephrons connect to the coelom via a ciliated peritoneal funnel, whereas medial nephrons lack this connection. Both nephron types are composed of a filtration unit, the Malpighian corpuscle, and a renal tubule, which can be divided into six morphologically distinct segments: neck segment, proximal tubule, intermediate segment, early distal tubule, late distal tubule, and collecting tubule. Collecting tubules merge and form a branch system that opens into collecting ducts. Collecting ducts empty into the Wolffian duct. Proximal tubules of nephrons in the frontal divisions are morphologically different from the proximal tubules of more caudal kidney regions. Distal tubule subdivision is only clearly recognizable at the electron microscopic level. The length of each nephron segment is calculated from a ventral nephron with a total length of ~3.8 mm, and the course of the segments within the nephric tissue is reported. The number of nephrons was estimated at 1,700 units in each kidney. The segmentation and ultrastructure of the mesonephric nephrons in G. seraphini are discussed in relation to nephron descriptions from other caecilians and we further discuss the evolutionary origin of the amphibian nephron. J. Morphol. 262:583-607, 2004. © 2004 Wiley-Liss, Inc.