The 'Trnjak' variety is a Croatian indigenous grape variety in which long term vegetative propagation has resulted in the appearance of high intra-varietal variability. During September 2019, a clone selection project was launched. A detailed ampelographic evaluation of phenotypically divergent clone candidates and sanitary status verification were performed. The yield, vigour, number of bunches per shoot, uvometric characteristics of the cluster and the grape, and sugar content were observed. The variety was identified within 10 locations in the Imotski vineyards by the mass positive selection. Visual inspection of positive phenotypic characteristics identified 133 vines as potential clonal candidates. Descriptive statistical methods determined the high divergence of the 'Trnjak' variety, from which those with admirable characteristics will be selected for the further propagation and for the clone registration of the same variety. Vine samples (basal part of the shoot) were subjected to serological ELISA (Enzyme-Linked Immuno-Sorbent Assay) to be analysed for the presence of four economically important viruses, Arabis Mosaic Virus (ArMV), Grapevine Fanleaf Virus (GFLV), Grapevine Leafroll-associated Virus 1 (GLRaV-1) and Grapevine Leafroll-associated Virus 3 (GLRaV-3). Out of the 133 samples collected, 18.80 % of the analyzed population of the variety is virus free. The rest of the samples were found to be infected, 31.58 % of them were infected by ArMV, 33.08 % by GFLV, 51.13 % by GLRaV-1 and 42.86 % by GLRaV-3.