An accurate, fast, and simple method for determination of thallium in environmental water samples with Zeeman electrothermal atomic absorption spectrometry (ZETAAS) by previous flotation preconcentration is given. The possibility of using cobalt(III) hexamethylenedithiocarbamate as a colloidal precipitate collector for simultaneous flotation separation of Tl(I) and Tl(III) from water matrices was studied. All experimental parameters influencing a proper flotation of both thallium ions were ascertained. The developed procedure with Co(HMDTC)(3) is applied for preconcentration and separation of total thallium in water matrix before ZETAAS. The results of ZETAAS analysis are compared with those obtained by inductively coupled plasma-atomic emission spectrometry. The ZETAAS limit of detection of thallium preconcentrated by Co(HMDTC)(3) is 0.031 mu g/L.