Abstract
The larva and pupa of a new species of black fly from the LaHave River in Nova Scotia, Canada, are described and illustrated. Simulium rothfelsi, n. sp., is most closely related to S. johannseni but can be distinguished from this species and all others in North America by the configuration of the pupal gill - two swollen dorsal filaments and two slender ventral filaments. This new species is univoltine, passing the winter as eggs and completing its larval and pupal development in a few weeks during May.