A seismo-volcanic event of unusual intensity and location occurred during a 6-day period beginning on Christmas Eve 1965 at Kilauea Volcano. This event, centered on the Kose fault zone, was marked by thousands of small earthquakes, extensive ground cracking, harmonic tremor, and a minor eruption on the east rift. The Kose fracture zone lies between the east rift zone and the southwest rift Zone of Kilauea, thus separating the south flank from the summit caldera. Historical records reveal a seismic gap between the rift zones. The Christmas 1965 activity filled this gap except near its eastern and western boundaries abutting the east and southwest rift zones. This event is different from other seismic gap-filling events in that (1) it was marked by an earthquake swarm rather than a main shock-aftershock sequence, (2) it was partly volcanic in nature, and (3) it did not occur on a plate boundary. This paper discusses this event with emphasis on its seismic character and its significance in terms of Kilauea tectonics and volcanism.