The morphology and morphometry of the gills of Oreochromis alcalicus grahami, a unique ureogenic teleost that lives in the alkaline environment of Lake Magadi, Kenya (pH 10, [Formula: see text], temperature 30 – 40 °C) were examined by transmission electron, scanning electron and light microscopy. Fish were examined in normal Lake Magadi water and 2 – 3 or 24 h after transfer to Lake Magadi water neutralized to pH 7 with HCl (i.e., [Formula: see text] replaced with Cl−), a treatment that caused severe reductions in urea excretion and O2uptake, internal acidosis, and ionoregulatory disturbance. In Lake Magadi water, the organization of the filament epithelium of the gill was similar to that of sea water teleosts. Indeed, chloride cells were located at the bottom of pits bordered by overlying pavement cells and flanked by typical accessory cells. Total numbers of chloride cells remained unchanged after transfer to pH 7, but after 2 – 3 h, many were covered by pavement cells, restricting their communication with the external milieu. At 24 h, this trend was reversed, an observation indicative of a reactivation of chloride cells. Mucous cells were located at maximum density on the trailing edge of the filament; most of them were empty after 24 h at pH 7. The harmonic mean thickness of the lamellar epithelium (blood-to-water diffusion pathway) was very small and not altered by acute or longer term exposure to pH 7. A model of alterations in ion and acid – base transport accompanying the morphological changes is presented.