A new RNAA procedure was developed capable of low level determination of thallium in biological and environmental samples. After high fluence neutron irradiation in a nuclear reactor, wet ashing of samples and T1(I) separation by solvent extraction with sodium diethyldithiocarbamate at pH 13, several types of counting were employed to compare their detection limits and to utilize the self-validation principle of NAA. The following measurement modes were used: High efficiency counting of γ-rays of202T1 and Hg X-rays produced on decay of204T1 using a well-type HPGe detector, combined γ− ray and β−-counting of204T1 with the aid of a HPGe planar detector, and liquid scintillation counting and counting of Cerenkov radiation of β−-particles of204T1. The lowest detection limit of 0.034 ng of T1 was achieved on liquid scintillation counting of204T1. The method was applied for the analysis of biological NIST SRMs 1515, 1573a, 1577b and environmental NIST SRM 1633a. Good agreement was found between the thallium certified value in SRM 1633a and values determined in this work by all counting modes. For SRM 1573a, results in agreement were obtained by two counting modes, while counting of Hg X-rays of204T1 was only used for SRMs 1515 and 1577b.