A simple and sensitive derivative spectrophotometric method has been developed for the determination of thallium(III) using a newly synthesized reagent diacetylmonoxime-(p-anisyl)-tliiosencarbazone in acetic acid medium. The molar absorptivity and Sandell's sensitivity of the coloured species are 5.6 x 10(4) L mol(-1) cm(-1) and 3.6 ng cm(-2), respectively. Beer's law is obeyed between 0.25 and 2.60 mu g mL(-1) of Tl(III) at 460 nm. The stoichiometry of the complex was found to be 1:2 (metal: ligand). Calibration graphs were derived by measuring first order derivative amplitudes at 425 and 505 nm. The metal ions like Cu(II), Cd(II), Pb(II), Zn(II) and Bi(III) which are usually associated with thallium containing samples do not interfere. The developed procedure has been successfully applied for the trace level determination of thallium in minerals, alloys, urine, soil and water samples.