Electrophoretic analyses of the tetrameric lactate dehydrogenase isozyme patterns of the endangered cyprinodontid fishAphanius fasciatuswere carried out. Organ and tissue samples of ten individuals of each sex were analysed by cellulose acetate elec‐trophoresis and isoelectric focusing. Isozyme electrophoretic patterns indicated that three LDH loci (Ldh‐A, Ldh‐BandLdh‐C)were active and showed differential tissue expressions. No differences between sexes were observed. TheLdh‐AandLdh‐Bloci were expressed in all tissues analysed, and the A subunits possessed a net negative charge greater than B subunits. Heterote‐tramers including subunit A were not observed, but a complex heterotetramer banding pattern of subunits B and C was detected. On the basis of the staining intensity of the electromorphs,Ldh‐Apredominated in all tissues analysed, with the exception of the liver, whereLdh‐Bpredominated. Instead,Ldh‐Cwas characterised by high relative anodal mobility and was expressed only in the eye. Furthermore, at this locus two alleles were observed in specimens analysed. The LDH banding pattern ofA. fasciatuswas consistent with that of advanced teleosts and it showed an inversion of the net negative charge of products of theLdh‐AandLdh‐Bloci.