Abstract.1The aphidsUroleucon tissotiandU.nigrotuberculatumboth exhibit significantly aggregated distribution patterns across clumps of their common host plantSolidago altissima, the tall goldenrod. However,U.nigrotuberculatumis significantly more aggregated than isU.tissoti. We tested the hypothesis that goldenrodgenotype‐by‐U.tissotigenotype interactions for aphid performance characters could partially account for this difference.2Only one of the characters we measured showed a significant genotype‐bygenotype interaction; therefore it is unlikely that such interactions affect the distribution ofU.tissotiacross goldenrod stems.3In addition, we found that three environmental variables (genetic variation for resistance to herbivory, host plant morphology, and local host density) affect the degree of aggregation found in bothU.tissotiandU.nigrotuberculatumpopula