Cold air temperatures may have killed several emerging nymphs and adult Pteronarcys californica on the Jocko River, Montana. Some aquatic insect taxa emerge early on the Jocko River, likely due to warm water temperatures in winter and spring. At this location, P. californica emerges approximately 5–7 weeks earlier than it does on many other streams or rivers across the United States. We hypothesize that this earlier emergence may be a mortality factor when cold fronts cause air temperatures to drop below freezing. Other mortality factors are also discussed.