The use of species’ traits is increasing in ecological research. Many studies obtain trait data from a single source, implicitly assuming the accuracy of these data. I critically evaluate this assumption by measuring agreement among sources for trait data. I evaluate inter-source agreement for 22 traits (anatomical, behavioural, life-history and niche-related) among five authoritative data sources (two field guides, two atlases and one online resource) for 263 Canadian butterfly species. This represents the first quantitative comparison of trait data among field guides or atlases. Traits varied considerably in their agreement among sources. Some traits such as wingspan and overwinter stage were fairly consistent among sources, whereas other traits such as habitat breadth were remarkably inconsistent among sources. These findings call into question the reliability of research that relies on a single source for trait data. I offer several recommendations for how trait researchers can account for inter-source variation in trait data.