Histological aspects of late gametogenesis in Leptodactylus labyrinthicus and of unfertilized oocytes collected from clutches in the field were studied by light microscopy. Specimens were collected during the reproductive period to determine why only 10% of the oocytes deposited in foam nests are fertilized. Sections of ovaries and oocytes were stained with hematoxylin and eosin, mercury bromophenol blue and toluidine blue. During the reproductive phase, the ovaries were completely developed and consisted of a sack-shaped, multilobular structure, with each lobe containing many oocytes in advanced developmental stages. Atretic oocytes were also seen in the ovaries during the reproductive phase. Oocyte development in the ovaries was considered synchronous, although few oocytes were seen in the early developmental stages. There were no differences in the morphology or staining of oocytes in the ovary and of unfertilized oocytes. Testicular development was synchronic with that of the ovary, with the testes also being fully developed during the reproductive period. Each seminiferous tubule had many cysts containing all of the phases of spermatogenesis, especially spermatids with different levels of nuclear condensation. Free spermatozoa were also observed in the lumen of the seminiferous tubule. The significant proportion of unfertilized oocytes present in many clutches may indicate that males produced an insufficient number of spermatozoa to fertilize all of the oocytes or that females deposited additional oocytes subsequent to spawning. These unfertilized oocytes are ingested by the larvae and may represent a reproductive strategy for increasing tadpole survival.