The green leaf area of winter barley, cv. Sonja, sampled from the field at different times during winter was always greatest in plants grown at high soil phosphate and smallest in plants grown at low soil phosphate, and at each fertilizer level was greater in healthy plants than in plants infected by rust (Puccinia hordei). In leaves that survived the coldest period of winter, the percentage area that was damaged was increased by rust infection which prevented the ameliorating effects of high soil P. Rust and low P interacted to reduce the increases in leaf area and shoot d. wt that occurred when higher temperatures prevailed in spring. Under controlled conditions in the laboratory, phosphate reduced the injury suffered when plants not acclimated to low temperatures were exposed to freezing conditions, but this effect was removed by rust infection. After rust infection, freezing temperatures were damaging even to acclimated plants, particularly if grown with low soil P. Evidence of visible symptoms, and quantitative measurements of electrolyte efflux from intact leaves, chlorophyll fluorescence in vivo, and ethane and ethylene evolution from cold-acclimated plants, showed that infection raised the minimum temperature at which tissues could survive without injury. Infected leaves were more sensitive to low temperature post-sporulation than presporulation. Measurements of electrolyte efflux and chlorophyll fluorescence on plants growing under cold conditions showed that infection inhibited the processes of acclimation to low temperatures.