Abstract. 1. Performances of two clones of Uroleucon caligatum on eleven clones of Solidago were measured by caging aphids on plants in the field.

2. Several measures were obtained, including developmental time from birth to adulthood, size of first instar nymphs, adult weight, and total colony weight.

3. All measures of performance were strongly affected by plant clone.

4. Effect of aphid clone-plant clone interaction was significant only for first instar size.

5. In a subsequent screenhouse experiment, plant clones were subjected to uniform conditions and still exhibited large differences in host quality.