Fatty acids added to cultures of Sarcina lutea caused changes in the hydrocarbon composition of the cells. These changes were consistent with a mechanism of synthesis in which the major fatty acid of the cells condensed with the added fatty acid or with fatty acids that increased in response to the added fatty acid and in which one of the fatty acids participating in the condensation was decarboxylated. In the presence of acetate in the medium, exogenous palmitate was incorporated into the hydrocarbon by a mode of entry in which it was specifically not decarboxylated. In media with low acetate, 60-70 % of exogenous palmitate incorporated into the hydrocarbon was decarboxylated. Under conditions of incorporation in which the palmitate was not decarboxylated, the carboxyl carbon of the palmitate occurred in monounsaturated hydrocarbons specifically on the side of the double bond opposite that in which the remainder of the aliphatic chain from palmitate was located. Evidence for the direct conversion of monounsaturated hydrocarbons into saturated derivatives and for the failure of ketones to serve as intermediates in the incorporation of fatty acids into hydrocarbons is presented. Alternative mechanisms for the intermediary conversion of fatty acids into hydrocarbons by the condensation of the carboxyl carbon and α-carbon of acids with decarboxylation of one of the acids (head-to-head condensation) that bypasses the requirement for ketones or secondary alcohols are presested. © 1969, American Chemical Society. All rights reserved.