Differentiation between scats of sympatric canid species is important for determining species-specific presence and movements, but distinction in the field is difficult. We calculated upper and lower thresholds of scat diameters to distinguish between scats of red wolves (Canis rufus) and scats of coyotes (C. latrans) and coyote–wolf hybrids in the field, in North Carolina, USA, from February 2009 to March 2010. We used DNA genotyping to identify scats collected in the field and took diameter measurements of those scats. Based on normal-distribution probability functions of scat diameters, scats ≥29 mm in diameter were ≥95% certain to be of red wolf origin. Conversely, scats ≤14 mm in diameter were 95% certain to be of coyote or hybrid origin. Scats >14 mm and <29 mm in diameter could not be identified by diameter alone. We suggest these upper and lower thresholds of scat diameters be used in concert with other methods (e.g., DNA genotyping) to monitor for red wolf, coyote, and hybrid activity to help conserve a lone, free-ranging population of wild red wolves. © 2011 The Wildlife Society.