Studie were conducted in 1978 and 1979 to evaluate the effects of varying twospotted spider mite populations on photosynthesis, transpiration, and productivy of strawberry plants under field conditions. Both photosynthesis and transpiration were significantly reduced by high levels of spider mite populations. Early season infestations caused reductions in physiological parameters at much lower population levels than was necessary to cause similar injury later in the season. Reduced physiological activity of mite-stressed plants persisted until plant recovery was observed following a rapid decline in spider mite populations. The number of fruit produced was significantly increased from plants experiencing low level stress from mite populations. After the initial increase, the number of fruit from infested plants decreased significanyly and the difference was greatest from plants that experienced early season stress. Fruit size was reduced significantly for all mite-stressed plants.