The phenomenon of liquefaction is investigated by comparing field evidence of an actual landslide with laboratory testing and analysis. The landslide occurred during the February 9, 1971 San Fernando earthquake at the site of a nearly completed water filtration plant in southern California. The cause of the landsliding was liquefaction of a natural deposit of saturated alluvium underlying a thick interval of compacted fill. Analytical and laboratory testing procedures are presented that demonstrate the validity of evaluating liquefaction potential during an earthquake.