Three sympatric aphid species of European origin (Euceraphis betulae (Koch),Callipterinella calliptera (Hartig) andBetulaphis brevipilosa Börner) feed onBetula pendula Roth in northern California. The component species of this guild partition resources by plant part, feeding sites within leaves and leaf phenological state. Overall niche overlap between species reached a maximum of 49.9% betweenB. brevipilosa andC. calliptera. Co-occurence ofE. betulae with both other species on leaves was random while co-occurrence ofC. calliptera andB. brevipilosa was aggregated. Morphological features (stylet length and body weight) act to mediate niche organization within this guild.California populations ofE. betulae andB. brevipilosa appear to utilize resources in a fashion similar to endemic Danish populations. Whereas DanishC. calliptera usually feed inside leaves silked together by spiders or Lepidoptera,C. calliptera in California also commonly feed on exposed leaf surfaces. This species, which has thus expanded its endemic niche, occupies the most generalized niche of the three species studied.