With good fruit characteristics, 'Siirt' is an important pistachio cultivar of Turkey. The harvest season of this cultivar usually is from early September to late September, The study was conducted in central Siirt province, origin of the cultivar 'Siirt', (the Southeastern Anatolia Region) during 1998 and 2000. This study determines the phenotypic differences related to nut and yield characteristics within the cultivar 'Siirt'. The data from nineteen selections grafted onto P. khinjuk and eleven selections grafted onto P. terebinthus were evaluated. The tree characteristics were determined for three years and nut characteristics were recorded for two years. The selections on P khinjuk showed a range of 0.52-6.80 kg for yield per tree, 11.9-67.1 g/cm(2) for yield efficiency, 103-118 g for dehulled nut weight, 50.7-59.4 g for kernel weight and 66-93% for split nuts. In addition, selections on P. terebinthus had a range of 0.70-2.84 kg for yield per tree, 7.5-29.1 g/cm(2) for yield efficiency, 98-130 g for dehulled nut weight, 53.4-60.6 g for kernel weight and 71-91% for split nuts.