According to Stoiber, Rose, Lange, and Birnie (1975, p. B-2), this volcano was "so continously active during the period from its birth in 1770 until 1958, that it assumed the title of 'lighthouse of the Pacific', Izalco has been nearly completely in repose since the latter date. Only a brief, very small flank eruption in 1966 has interrupted dormancy. The extrusion of a small (900,000 m3) lava flow from a vent 550 m below the summit had remarkable little effect on the summit area, and Strombolian activity, in the past a hallmark of Izalco, was completely absent. Details of this eruption are contained in a pervious article (Rose and Stoiber, 1969). Since 1964 temperatures of the fumaroles in the summit crater of Izalco have been measured periodically, and a variety of other observations have been made irregularly."
The table below is a listing from Volcanoes of the World by Simkin and Siebert illustrating the eruption history of Izalco volcano.