Commercially grown canola, Brassica napus, is negatively affected by both water deficiency and herbivory. A glasshouse experiment evaluated the relationship between water deficiency and herbivory of two aphid species, Brevicoryne brassicae (L.) and Lipaphis erysimi (Kaltenbach) (Hemiptera: Aphididae), on canola. The effect of water deficiency on aphid performance was also evaluated. Brassica napus plants were provided with either continuous water (well-watered) or were watered only once per week (water-deficient). The plants were exposed to either B. brassicae, L. erysimi, or no aphids at all. Aphid population increase, plant growth, and water relations were measured after 3 weeks. Water deficiency reduced plant growth and altered water relations in B. napus. Feeding by the aphid L. erysimi substantially reduced growth of both water-deficient and well-watered B. napus. However, herbivory by B. brassicae reduced plant growth only in conjunction with water limitation, demonstrating an interaction between limited water and insect infestation on the performance of B. napus. The aphid species and water deficiency treatments also differentially affected turgor potential, osmotic potential, leaf water potential, and leaf water content. The two species of aphids responded differently to changes in their host plants. The average size and numbers of B. brassicae aphids were lower but the percentage of alates was higher on plants with limited water relative to those on well-watered plants. The average size and numbers of L. erysimi aphids were unaffected by water limitation and the percentage of alates produced was slightly reduced in the water-deficient plants. Water limitation is likely to reduce the growth of plants, particularly when B. brassicae infests the crop. However, L. erysimi is probably the more serious pest of the two aphids in that it reduced plant growth regardless of watering regime and its development was unaffected by the status of the host plant.