A sample of thin-lipped grey mullet, Liza ramada (Risso, 1826), from the River Tagus was examined for the purpose of age determination. The relation between the fish standard length (SL) and the total scale radius (SR) was: SL = 33.0 SR + 22.4. The following characterisics were established: that the scales that covered the mullet's body, near the pectoral fin, made their first appearance at a standard length of 22.4 mm; that annuli represented year marks, laid between January and February and; that as the eclosion period did not match the season during which the annuli were laid, there was a one to six months difference between the fish real age and the age deduced by scale examination. von Bertalanffy growth equations for the River Tagus thin-lipped grey mullet population are: males: SL_t = 393.5 [1 - e^{-0.13 (t + 1.6)}]; females: SL_t = 457.9 [1 - e^{-0.12 (t+ 1.5)}] In fish with a SL larger than 288.6 mm the first annulus may be hiden by the scale calotte.