We investigated the response of four species of aphids (Metopeurum fuscoviride, Brachycaudus cardui, Aphis fabae, and Macrosiphoniella tanacetaria) on tansy (Tanacetum vulgare) to plant quality and attendance by an ant, Lasius niger. The aphids experienced one of four different environments for two consecutive generations. Ant-attendance significantly affected the time needed to reach maximum fecundity only in Me. fuscoviride and plant quality in Me. fuscoviride and B. cardui. Maximum daily fecundity was positively affected by plant quality and the magnitude of the effect was inversely associated with the degree of myrmecophily. Ant-attendance had a positive effect on maximum fecundity only in the obligate myrmecophile, Me. fuscoviride. The intrinsic rate of population increase, rm, on high quality plants, was lowest for the obligate myrmecophile, intermediate for the facultative myrmecophiles and highest for the unattended species. On high quality plants the fitness of Me. fuscoviride was more adversely affected by the developmental stage of the plant and absence of ants than that of A. fabae or Ma. tanacetaria, which were able to maintain a high relative fitness in all the environments. The implications for aphids experiencing different degrees of ant-attendance and seasonal changes in plant quality are discussed.