Conspecific aggression may play an important role in partitioning resources and maintaining territories among beavers (Castor canadensis), yet few studies have examined physical evidence of agonistic encounters. We trapped and examined pelts from 147 beavers harvested between 2006 and 2012 from the Sangamon River (n  =  96) and tributary streams (n  =  51) in central Illinois. We modeled the influence of sex, age class, season (predispersal or dispersal), and habitat (river or tributary stream) on the number of recent injuries caused by conspecifics. One-third (51/147) of beavers had ≥1 injury; of those, the median number of injuries was 2.0. Kits had fewer injuries than adults (β_Kit  =  −2.24 ± 0.63), but yearlings and subadults did not (β_yearling  =  0.02 ± 0.38, β_subadult  =  −0.22 ± 0.48). Beavers on small streams had only one-quarter of the injuries recorded for beavers on the river (β_Stream  =  −1.34 ± 0.82). We failed to detect differences in injuries between the sexes. Our results suggest both sexes participate in territorial defense through physical confrontations and such encounters can be costly to both dispersing juveniles and resident adults.