The strength of herbivore suppression often improves with greater predator biodiversity, but controversy remains about the ecological mechanisms underlying such results. Positive predator diversity effects might result from partitioning of the prey resource, with different predator species attacking different subsets of the prey population. In a large-scale field experiment we manipulated diversity among a community of predators, at two densities, and measured the impact of these manipulations on aphid prey. Aphid suppression always improved with greater predator diversity. However, the magnitude of this positive diversity effect was greater at high predator density resulting in a significant predator density by diversity interaction. Low density monocultures performed no differently than the high density monocultures suggesting that each individual predator’s unique “niche space” was completely filled at the low density. These data are consistent with what would be expected if niche partitioning was the underlying mechanism for a positive diversity effect.