Abstract Twenty natural hybrids between yellow birch, Betula alleghaniensis , and paper birch, B. papyrifera , are reported from Michigan, Minnesota, and New Hampshire. The seed parent is apparently B. alleghaniensis. Hybridization is likely where the parents occur together, and in these areas the production of hybrid seeds is apparently widespread. The establishment of hybrid seedlings is most common on disturbed, mesic sites along roadsides and in cut-overs. Hybrids are distinguished by their bark color and texture, fruit characters, and the number of leaves borne on short shoots. Single-variate and multivariate analyses showed the hybrids were intermediate between the parents in leaf characters but resembled B. alleghaniensis more closely in fruit traits. The taxa do not differ in pollen size and abortion percentage. Seeds of two hybrids were germinable. Forest Sci. 20:215-221.