Previous research has demonstrated that beavers (Castor canadensis) dramatically alter fish habitat in streams by their dam-building activities. Although less well-known, beavers also flood closed peatlands by damming seepage rather than streamflow. Our study focuses on the establishment of fish communities in isolated beaver ponds created in small, ombrogenous peatlands lacking any open water prior to beaver occupation. We selected 16 ponds that ranged in age from 4 to 42 years and three unaltered peatlands to determine whether beaver promote the use of peatlands by fish and if the patterns of individual species were related to macrophyte density. Fish were present in ponds as young as four years old, and five of the six species of fish were present in ponds ≤16 years old. Submersed macrophyte abundance explained 40% of the variation in fish diversity. Rapid colonization of fish in beaver-altered peatlands provides further evidence of how beavers increase the complexity of biological food webs in boreal regions. Moreover, this work provides an even greater understanding of the historical role of beavers as ecosystem engineers in peatland dominated landscapes.