Archips negundanus Dyar occurs widely in the United States and Canada and feeds on a variety of plants. In Utah, frequent infestations of boxelder (Acer negundo) have been recorded since 1931. A current outbreak in the northern portion of the State provided an opportunity to study the biology of this leafroller. A. negundanus is univoltine and overwintered in the egg stage. The larvae hatched in May, and most completed development by mid-June. The head-capsule widths of field-collected larvae indicated five instars. Pupation occurred during June and July in rolled leaves and to a lesser extent in fruit racemes. The pupal stage averaged 10 days in the laboratory at 23 deg C. Adults appeared in mid-June in the field, 11 days after the first pupae were sighted. The flight period extended into July. A list of the parasites reared is given.