I designed laboratory and mesocosm experiments to test the hypotheses that consumption of a congener as supplemental food can increase the size variance, size, aggression rates, and mortality of intraguild predator populations. Experimental populations of Marbled Salamander (Ambystoma opacum), Jefferson Salamander (Ambystoma jeffersonianum) and Tiger Salamander (Ambystoma tigrinum) larvae that were initially fed a smaller congener Spotted Salamander (Ambystoma maculatum) as a food supplement developed larger SVL and size variation after seven days than control larvae that were not fed a congener. Experimental treatment populations had greater initial size variation because some larvae ate a congener and others did not. Treatment larvae had increased SVL, aggression rates and mortality after 60 days compared to control populations. These results suggest that intraguild predation on a congener can affect fitness and population dynamics of predator populations. I hypothesize that intraguild predation on congeners results in size variation and then increased aggression and mortality caused by cannibalism.