Red raspberry (Rubus idaeus L. cv. Willamette) primocanes were artificially defoliated to various degrees and at two dates in each of 2 years to simulate defoliation caused by the twospotted spider mite (Tetranychus urticae Koch). The effect on primocane diameter, lateral length, yield and four yield components was determined. When defoliation occurred evenly along the length of the primocanes, and all floricanes and excess primocanes were removed in early Aug. 1989, yield was reduced 26% at 25%, 50%, and 75% defoliation and 55% at 100% defoliation compared with nondefoliated controls. The number of canes per stool, number of fruit per lateral, and weight per fruit were reduced when defoliation occurred earlier, in August rather than September, but the number of laterals per cane increased with early defoliation. The effect of increasing defoliation on plant growth and yield was generally nonlinear. When defoliation occurred in sections along the lower 2 m of all primocanes, and all floricanes and excess primocanes were removed in Nov. 1992, no significant differences in yield or three yield components were detected. The effects of primocane defoliation are not necessarily predictable, so T. urticae should be controlled before mite-induced defoliation occurs.