Abstract The incorporation of l-[3- 14C] serine into the phosphatides of Musca domestica larvae was examined in vivo, and of this compound and [2- 14C] ethanolamine in vitro using whole fat body preparations. Rapid incorporation, probably by an exchange reaction, was observed in both cases. Lipid serine was the direct precursor of lipid ethanolamine over the short time periods studied. Ethanolamine depressed the incorporation of serine but the reverse effect was not shown. These results are discussed in relation to current ideas of mammalian phosphatide metabolism.