During two successive years 1997 and 1998, clusters of Thompson Seedless grapevines were subjected to sitofex and/or GA3 spray when berry diameter reached 4-6 mm.  Sitofex treatments were 0, 3, 5 and 7 ppm where GA3 was sprayed with 0 or 40 ppm once or twice.  Sitofex and/or GA3 significantly increased berry length, diameter, weight, size, firmness, adherence strength, cluster weight, yield, acidity and Ca in berry skin where TSS and TSS/acid ratio were significantly decreased.  Sitofex delayed grape maturation more than GA3 and the combined application delayed maturation more than sitofex or GA3 alone.  It is recommended to spray clusters when berry diameter reaches   4-6 mm  with 40 ppm GA3 + 5 ppm sitofex + 40 ppm GA3 (after 5 days), to produce high quality Thompson Seedless grapes.