Communal parasitism in galling aphids is reported and the effects of gall invasion by the inquiline aphid Tamalia inquilina Miller on its host, Tamalia coweni (Cockerell) are quantified. On populations of the host plants Arctostaphylos spp.(Ericaceae), both T. coweni and T. inquilina exhibited facultatively communal behavior and co-occupied galls with no apparent agonistic interactions. Although total reproductive output of adult offspring was similar between galls containing T. coweni alone and galls with both species, the allocation of brood was significantly skewed toward that of the inquiline; hence, T. inquilina is a parasite of T. coweni. However, the presence of T. inquilina had no significant effect on survivorship of T. coweni in mixed-species galls. Implications of this inter-specific interaction are discussed in light of the evolution of social parasitism.