<p>The Magadi tilapia thrives in arguably the most extreme aquatic environment on earth for fish, the hot springs of Lake Magadi in Kenya with its severe water chemistry: pH 10, alkalinity 300&#xa0;mEq·L⁻¹. This fish is 100% ureotelic yet has an osmoregulation pattern typical of marine teleosts, although the dominant water&#xa0;anion is HCO₃⁻ rather than Cl⁻. The gills must actively export base equivalents (HCO₃⁻ + CO₃²⁻) and Na⁺ against strong electrochemical gradients, yet simultaneously take up Cl⁻, for which a hypothetical “Laurent model” based on ionocyte structure alone was proposed. This model has been tested using immunohistochemistry to characterize ionocyte types based on ion transport protein expression patterns [CFTR anion channel, Na⁺:K⁺:2Cl- cotransporter (NKCC)/Na⁺:Cl⁻ cotransporter (NCC), Na⁺:HCO₃⁻ co-transporter (NBC), Na⁺/K⁺-ATPase (NKA), and urea transporter (UT)]. A typical “seawater ionocyte” (Type IV) with apical CFTR and basolateral NKCC1 and NKA, is present validating key elements of the model. A “freshwater ionocyte” (Type II) is also present (apical NCC, weaker NKA and strong NBCe1 basolateral staining). A third Type I ionocyte with only NKA staining was also identified. An acid excreting Type III ionocyte (apical NHE3 and basolateral NKA) was not present. The Magadi tilapia is unusual in having co-expression of both Type IV and Type II ionocytes, which are typically associated with Cl⁻ excretion and uptake, respectively. Instead, we propose Type IV ionocytes are involved in base equivalent and Na⁺ excretion and Type II ionocytes in Cl⁻ uptake. In these ureotelic fishes, the UT occurs only in lamellar pavement cells.</p>