Predators have been widely demonstrated to influence developmental plasticity in aquatic prey. We investigated the body size of the amphipod Hyalella azteca raised with predatory sunfish by exposing solitary or groups of three amphipods to sunfish kairomones or a control for 30 days. Field studies indicate that small body size in H. azteca is likely an adaptive response to size-selective sunfish predation. Grouped amphipods chronically exposed to the kairomones had significantly smaller body lengths than grouped amphipods exposed to controls, while individual amphipods exhibited no significant differences. Behavioral trials revealed that grouped amphipods sought refuge less than solitary amphipods. To examine if a conspecific chemical cue was related to the reduction in body size of the grouped amphipods exposed to kairomones, individual amphipods were exposed to a combination of conspecific chemical cues and kairomones or to controls for 30 days. Amphipods exposed to kairomones in combination with conspecific cues had significantly smaller body length than amphipods exposed to conspecific cues alone. These results mirrored the group versus individual exposure experiment, suggesting that a conspecific chemical cue is required to induce an anti-predator response. The present study highlights the importance of conspecific cues for plastic responses to predators.