AbstractWe describe morphological sex differences in urinary bladders of the urodele amphibianNecturus maculosus. The mucosal epithelial cells of bladders from males were tall and contained considerable PAS‐positive material. In scanning electron micrographs of the mucosal surface, epithelial cells from male bladders were well demarcated and were capped with microplicae or with long cilia. The mucosal epithelial cells of bladders from females were low and contained only a small amount of PAS‐positive material; in scanning electron micrographs cell boundaries could not be distinguished and no cilia or microplicae were present. Bladders from males had higher transepithelial potential difference and lower water content than bladders from females. Urine analyses were not significantly different in the two sexes. It is suggested that response to androgens in the male accounts for the observed differen