Seasonal average increases of 12% to 18% in fruit yield have resulted from experimental application of growth regulator sprays on blackberries. The experiments were conducted on the Boysen, Olallie and Thornless Logan varieties at Torrey Pines in 1955. Preliminary experiments completed a year earlier on the Olallie and Boysen varieties in Santa Cruz and Stanislaus counties had given some evidence that growth regulator sprays were effective in this respect.