Nuclear ribosomal internal transcribed spacer (ITS) regions were used to address questions about the taxonomy, phylogeny, and biogeography of the hazelnut genus, Corylus (Betulaceae). The ITS phylogeny supports the recognition of two sections within the genus: section Acanthochlamys comprising the basal extant lineage Corylus ferox, and section Corylus consisting of the remainder of the genus. Within section Corylus, recognition of three subsections is supported: subsection Corylus (species with leafy, more or less bell-shaped involucres), subsection Colurnae (tree species with deeply dissected involucres), and subsection Siphonochlamys (species with tubular involucres). The ITS sequence divergence among species within Corylus is low, corresponding with that of most other North Temperate disjunct taxa. A few morphological characters were added to the analysis to boost support for branches. Migration between Asia and North America via the Bering land bridge is suggested by subsection Siphonochlamys while long distance dispersal from Europe to North America during the late Tertiary is suggested by the close relationship and lack of sequence divergence among members of subsection Corylus. A close biogeographic relationship among areas of eastern Asia, the Himalayas, and the Mediterranean region is suggested by the subsection Colurnae clade.

Communicating Editor: Matt Lavin