The aim of the current investigation is to study the embryogenesis of the pseudobranchia
(PB) in the Eastern mosquito fish Gambusia holbrooki, Girard, 1859 (Cyprinodontiformes,
Poeciliidae). Histological sections from 2-4 mm long embryos revealed the presumable PB as
a cell mass surrounded by a row of squamous cells in the operculum, as well as the formation
of cartilage and blood vessels, and the beginning of the formation of its lamellae. In 5-7.75
mm long embryos, a prominent rise appeared in the mass of cells and PB lamellae, along with
an increase in melanin deposition around the blood vessels. Moreover, a row of squamous
cells encircled the rise from the front and back. At this stage, the parallel arrangement of the
cartilage-attached lamellae correlated with increased melanin concentration around the blood
vessels. In newborns, the differentiated cells increased similar to that appearing in the adult’s
stages.