Gravid females ofRhopalomyia solidaginisLw. oviposit on the vegetative shoot buds ofSolidago altissimaL., and their emerging larvae induce the development of conspicuous, rosette galls with varying numbers of larval chambers per gall. Most galls have only one larval chamber, whereas some have 2-12 chambers. These differences in the number of larval chambers per gall result in variability in the dimensions of galls. Gall populations from several old fields of central Pennsylvania were harvested to assess the morphometric relationships within galls of various numbers of larval chambers, energy allocation pattern among gall components, and host-plant energy allocation to the galls. We found that the normal stem diameter, a measure of host-plant vigor, is a predictor of gallmaker performance. Also, gall sizes likely vary because the gall midges differ in their ability to induce galls but primarily because of variation in the number of larvae per gall and in the quality of different host genotypes. We also found that the strong relationship between the number of larval chambers and the gall stem diameter indicated the stimulation of the goldenrod genotype to create a strong resource base for accommodating the gall. However, our results suggest the possibility of competition among siblings (when the gall houses multiple larvae) for host-plant resources. Our energy estimates reinforce the role of host-plant quality in determining the gall quality.