Canada's Species at Risk Act (SARA) and the US Endangered Species Act (ESA) have adopted different approaches to achieve overlapping goals. We compare the ESA and SARA, focusing on the roles of science and policy in determining which species warrant legal protection. Our analysis suggests that each act could benefit from mimicking the strengths of the other, and both could be strengthened by greater clarity and transparency of listing determinations. A particular strength of SARA is that all evaluations of species' status are conducted by a single national scientific body. The ESA does not involve a comparable national body but has more stringent legal deadlines for listing actions, and listing decisions cannot by law consider socioeconomic factors (as can occur under SARA). The conservation of biodiversity would be enhanced if both acts were complemented by additional programs focused on broader efforts that protect more species before individual intervention is needed.