The kidney of the pufferfish Sphoeroides testudineus, an abundant tropical euryhaline estuarine species of the western Atlantic Ocean found in southern Brazil (in salinities ranging from 0 to 34) has a large and laterally spread cranial red portion, and a very thin and pale caudal portion. When studied under light and transmission electron microscopy, the cranial kidney displayed glomeruli and renal tubules surrounded by haematopoietic tissue. These tubules appeared to drain into a single large convoluted collecting duct with a wide lumen and thick pseudostratified epithelium, the mesonephric duct, which constituted the sole structure of the caudal kidney. Apical microvillae were viewed in the renal tubules, as well as in the mesonephric duct. Basal mitochondria and membrane infoldings were observed in the renal tubules. Abundant more basally-located mitochondria and electron-dense vesicles, mainly in the apical cytoplasm, were observed along the entire length of the mesonephric duct. Aposomes (blebs) were frequently observed in the mesonephric duct, both by light- and electron-microscopy. This euryhaline estuarine pufferfish has thus been revealed to possess a rare type of kidney.