Abstract: Size-specific interactions add a great deal of complexity to traditional views of food webs and trophic levels. An understanding of size-specific interactions, how they work, and their ecosystem level effects are, therefore, an important step towards understanding how the natural world functions. This study deals with two key community members in headwater streams and the size-mediated way in which they interact. The crayfish ( Cambarus bartonii ) and larvae of the spring salamander ( Gyrinophilus porphyriticus ) have a great deal of niche overlap and both are very abundant in headwater streams. The first aspect of this study was to examine naturally occurring patterns of body size, total biomass and abundance in ten streams. There were no negative correlations between any combinations of these factors, demonstrating that intraguild predation is not the dominant factor in structuring populations. Two paired experiments are currently underway to determine whether a large individual of one species has an effect on the growth and/or survivorship of a size gradient of the second species. A significant interaction in the experiment will suggest the possibility of intraguild predation in natural populations, even though other factors seem to have a more important role in structuring these populations.