Continuous monthly mass balance measurements from the ablation zone of Antizana 15 glacier in the Andes of Ecuador between January 1995 and December 2002 indicate a strong dependence on El Ni¿o--Southern Oscillation (ENSO). Over the 8-year period investigated, mass balance was negative all year round during El Ni¿o periods but remained close to equilibrium (positive anomalies) during La Ni¿a events. On seasonal timescales, mean ablation rates remain at a quite constant level all year round, but interannual variability shows much larger changes from year to year during the key periods February--May and September. This variability is caused by large differences that occur in the seasonal cycle during the two opposite phases of ENSO. Since ENSO is phase locked to the seasonal cycle with largest sea surface temperature anomalies around boreal winter, November--February, and the atmospheric response to ENSO is delayed by 3 months over the Ecuadorian Andes, year-to-year variations in mass balance are largest between February and May. Energy balance studies at the glacier surface indicate that high air temperature, which favors rain over snowfall, weak and sporadic snowfall, insufficient to maintain a high glacier albedo, low wind speeds, which limit the transfer of energy from melting to sublimation, and reduced cloud cover, which increases the incoming short-wave radiation, are the dominant factors related to El Ni¿o, which tend to increase ablation. La Ni¿a events on the other hand are characterized by colder temperatures, higher snowfall amounts, and to a lesser degree, more constant winds, factors which increase albedo and sublimation and therefore preclude melting at the glacier surface. The effects of ENSO variability are also important over the accumulation area, which represents up to 80% of the glacier surface during La Ni¿a events (1999--2000) and 45--60% in El Ni¿o years. Since the accumulation rates increase during these cold periods, the specific net balance and the dynamics of the entire glacier are strongly affected. Longer mass balance records than this 8-year period are needed for conclusive answers about the dependence of the Ecuadorian glaciers on ENSO variability, but initial results suggest that the response observed on Antizana glaciers is very similar to what has been observed previously during ENSO periods on Andean glaciers in the outer tropics. The seasonal dependence on ENSO and the physical mechanisms linking ENSO with mass balance variations on Antizana, however, are different from the response observed on Andean glaciers in the outer tropics.