Larvae of the gall-inducing moth Gnorimoschema gallaesolidaginis (Riley) (Lepidoptera: Gelechiidae) attack ramets of Solidago altissima L. and S. gigantea Aiton (Asteraceae), initiating stem galls early in ramet growth. We examined the relationship between ramet size (as an indicator of plant vigour) and galling rate over 3 years at a field site in Toronto, Ontario, Canada. We marked Solidago ramets along line transects, measured their stem diameter, and recorded their fate (galled or ungalled) during the season. For S. altissima, galls were numerous enough for analysis in 2 years, and the frequency of galling increased monotonically with ramet stem diameter in both years. For S. gigantea, galls were numerous enough for analysis in all 3 years, but attack rate - stem diameter relationships were complex. In 2004 the galling frequency peaked at intermediate stem diameter, but in 2005 the galling frequency increased monotonically with stem diameter (and in 2006 the nonsignificant trend was similar). Overall, our data are most consistent with the plant-vigour hypothesis, but the 2004 data for S. gigantea lend some support to the suggestion that herbivore attack might sometimes be most intense on intermediate-sized modules.