When chick liver cells in monolayer culture were incubated with 32Pi in the presence of insulin, acetyl-CoA carboxylase became extensively labeled with 32Pi reaching a stoichiometry of 9 to 10 mol of phosphoryl group per mol of 240,000-dalton enzyme subunit. The covalently bound phosphate was found to be metabolically labile, turning over with a t1/2 of approximately 2 h (enzyme t1/2 approximately equal to 24 h). Addition of Bt2cAMP altered neither the rate nor extent of phosphorylation. Contrary to other reports, the fully phosphorylated acetyl-CoA carboxylase appears to be catalytically active.