Dormancy of white ash (Fraxinus americana L.) and green ash (Fraxinus pennsylvanica Marshall) seeds with pericarps removed was studied in petri dishes and in a peat-based medium in the greenhouse. When dormant, surface disinfested seeds with one-third of their length excised from the cotyledon end were placed in petri dishes, white ash seeds displayed only cotyledon growth, whereas cotyledons, hypocotyls and radicles of green ash seeds elongated. Seeds of both species were cut into four transverse segments, and placed in petri dishes where the depth of dormancy was shown to decrease in a significant linear manner from the radicle through the cotyledon tips. When dormant seeds with the cotyledon ends excised were sown in a peat-based medium in the greenhouse, green ash seeds emerged and survived at levels significantly greater than white ash seeds. Green ash seeds that were cut transversely showed evidence of being negatively photoblastic, whereas intact green and white ash seeds showed no evidence of being either positively or negatively photoblastic. These results indicate that white ash has a deeper, more complicated dormancy than green ash, consistent with germination inhibitors in the embryo, seed coat and/or endosperm. Dormancy of neither species appears to involve immature embryos.