Squalene synthetase was solubilized from yeast microsomal membranes with deoxycholate. Solubilized enzyme was associated with one or more proteins with s20, w = 3.3 S, Stokes' radius = 40 A, and computed molecular weight = 54,500. In the presence of detergent the enzyme was catalytically inactive and unstable to heat. When detergent was removed with cholestyramine resin, both phases of squalene synthesis (farnesyl pyrophosphate leads to presqualene pyrophosphate leads to squalene) were recovered, and the enzyme was reaggregated to form sedimentable particles with a density of approximately 1.16 g/ml. Both activities were lost to variable extent upon chromatography over Sephadex G-200 in the presence of 0.2% deoxycholate, but could be recovered if phosphatidylcholine or phosphatidylethanolamine (but not phosphatidylserine or phosphatidylinositol) were added to fractions before removal of detergent. There was an apparently absolute requirement for phospholipid by the enzyme. The proteins catalyzing the two phases of squalene synthesis could not be resolved from one another and behaved in an identical fashion throughout a variety of manipulations.