Eurosta solidaginis(Fitch) infesting the host plantSolidago giganteaAit form larger galls than individuals attacking the presumed ancestral host,S. altissimaL. Because previous studies found thatEurytoma giganteaWalsh parasitism is restricted to smaller galls, it was hypothesized that this gall-size difference could result in decreased parasitism ofE. solidaginisinfestingS. gigantea. E. solidaginisgalls were collected along a transect from Pennsylvania to Minnesota and from two sites in Iowa to determineE. solidaginissurvivorship and causes of mortality. AlthoughS. giganteagalls were generally larger thanS. altissimagalls, there was no significant decrease inE. giganteaparasitism forE. solidaginisoccupyingS. giganteagalls. Galls containing the parasitoidE. giganteawere significantly larger on the host plantS. giganteathan onS. altissima. Parasitoid access toE. solidaginiswas not limited by the larger size ofS. gigallteagalls.Eurytoma obtusiventrisGahan parasitism was restricted to galls onS. altissimaand this parasitoid occurred only in the more eastern parts of our transect. Differences in gallmaker survivorship and natural enemy occurrence betweenSolidagohosts at sympatric sites were dependent on site; predation by the beetleMordellistena unicolorLec. was the primmy determinant of differences in gallmaker survivorship betweenSolidagohosts and among geographic sites.