Here we report a novel separation technique for the clinically important ¹⁹⁹Tl radionuclide from its precursor lead and daughter product mercury. Two algal genera, Lyngbya major, from cyanophyceae and Rhizoclonium hieroglyphicum from chlorophyceae have been used as natural bioreagents in the separation of thallium radionuclides in ultra-trace scale. 24 h algal treatment of an aqueous solution at pH 8 containing a mixture of radionuclides shows that Rhizoclonium accumulates 100% Hg, >80% Pb and a small fraction of thallium (15%). Thus a large fraction of thallium remains in the solution not accumulated by algae. Thus the selectivity of various algae towards different metals in sorption and desorption processes may be exploited in the separation of these metals from a mixture of them.