Beavers (Castor canadensis) exhibit wide variations in colony composition and demographics over their broad geographic range, so regional population studies are important for sound management of this species. The objectives of this study were to investigate the: (1) size and sex-age composition of beaver colonies in Illinois, (2) reproductive potential of female beavers in these colonies and (3) efficacy of night-vision surveys versus removal trapping for estimating colony size. We harvested and aged 239 beavers (128 males:111 females) during the 1999–2000 and 2000–2001 trapping seasons. The average colony contained 5.6 beavers. Family groups consisting of a breeding pair and at least 1 offspring composed 86% of these colonies; the other 14% consisted of only a breeding pair. Samples of beavers harvested by commercial trappers were skewed towards yearlings and 2-y olds relative to samples taken from trapped-out colonies. Fetal rates were 3.0, 3.4 and 4.2/female for yearlings, 2-y olds and older adults, respectively. In utero loss was estimated as 13%. Our estimates suggest that over 50% of kits die during the first 6 mo of life. We found no evidence that the presence of older offspring in a colony allowed parents to raise more kits. Night-vision surveys conducted 10 m downwind from the den for 2.5 h after sunset underestimated the size of colonies, accounting for only 55% of the beavers present.