Few researchers have used radiotelemetry to study spatial organization in beavers (Castor canadensis) given limitations in available radio-attachment methods for the species. During Mar. 2005–Sept. 2006, we attached tail-mounted transmitters to 46 beavers from 10 colonies in an unexploited beaver population in southern Illinois. We used radiotelemetry to quantify sizes and percentage overlap of composite and seasonal home ranges and core areas of beavers and tested for differences by age and sex. Composite home ranges and core areas averaged 25.5 ha and 3.9 ha in size, respectively, and did not differ between age and sex (P > 0.41). There was no difference in seasonal home range sizes between age or sex (P > 0.14); however, pooled home range sizes differed by season and year (P = 0.007). Seasonal core area size did not differ among age, sex, or season (P > 0.10). Percentage of seasonal home range overlap approached significance among seasons (P = 0.077), and seasonal core area overlap differed by season (P = 0.005). Mean composite colony home ranges and core areas were 22.7 ha and 3.5 ha in size, respectively. Nine out of 11 colonies had overlapping home ranges with ≥1 neighboring colony (x ¯ overlap = 9.8 ± 1.5%) and to our knowledge represents the only known documented colony home range overlap in this species.