1. The population dynamics of colonies of nettle aphids (Microlophium carnosum Buckton) were studied on potted nettle plants placed beside plots of grass on some of which outbreaks of grass aphids (Rhopalosiphum padi L.) had been induced by fertilizer application. 2. Nettle aphid colonies adjacent to grass aphid concentration suffered an earlier population decline, and produced fewer alate dispersers, than control colonies. 3. The reduced performance of nettle aphids in the vicinity of grass aphid was due to increased predation by Coccinellidae, attracted into the area by the large concentrations of grass aphids. The indirect interaction between the two species of aphids is an example of apparent competition. 4. Because predators pre-emptively exploited nettle aphids on plants in the grass aphid treatment, the numbers of nettle aphids attacked by parasitoid wasps was greatly reduced in these sites compared to the controls.