Liquefaction effects generated by the 1977 San Juan Province, Argentina, earthquake (Ms = 7.4) are described. The larger and more abundant effects were concentrated in the 60-km long band of the lowlands in the Valle del Bermejo and in an equally long band along the Rio San Juan in the Valle de Tulum. Fissures in the Valle del Bermejo were up to several hundred meters long and up to several meters wide. Sand deposits, from boils that erupted through the fissures, covered areas up to tens of square meters. Fissures generally parallelled nearby stream channels. Because the Valle del Bermejo is undeveloped, these large features caused no damage. Liquefaction in the Valle del Tulum caused important or unusual damage at several localities, including the following five sites: (1) At the Barrio Justo P. Castro, a subdivision of Caucete, liquefaction of subsurface sediments decoupled overlying, unliquefied stiff sediments, producing a form of ground failure called "ground oscillation". The associated differential ground movements pulled apart houses and pavements in extension, while shearing curbs and buckling canal linings in compression at the same locality. (2) At the Escuela Normal, in Caucete, the roof of a 30-m long single-story classroom building shifted westward relative to the foundation. That displacement fractured and tilted columns supporting the roof. The foundation was fractured at several places, leaving open cracks, as wide as 15 mm. The cumulative width of the open cracks was 48 mm, an amount roughly equivalent to the 63 mm of offset between the roof and foundation at the east end of the building. The ground and foundation beneath the building extended (or spread) laterally opening cracks and lengthening the foundation while the roof remained in place. (3) The most spectacular damage to structures at the community of San Martin was the tilting of a 6-m high water tower and the toppling of a nearby pump house into a 1-m deep crater. Similarly, a small crater developed beneath a hand-pump in an open area and a large, 6-m diameter crater formed nearby. The following sequence of events created the craters and toppled the pump structures: During the earthquake, ground shaking generated excess pore pressures which were dissipated by upward flow of groundwater. Free drainage was restricted by an impermeable plastic-silt layer. Water apparently accumulated below the plastic-silt layer and then burst to the surface through several holes and cracks, including holes around well casings. (4) At the San Isidro winery, nine storage tanks tilted 2 to 5??. Five reinforced-concrete tanks were dismantled but four steel tanks were repaired by placing new footings and jacking the structures into an upright position. (5) At Escuela J.J. Pasos, differential settlement beneath building fractured several columns and walls. The largest settlements were about 60 mm and the maximum settlement of footings supporting columns was about 40 mm. In spite of the damage, the buildings were in no danger of collapse.