In Torpedo marmorata electroplaque, an extrinsic membrane protein of apparent mass 43,000 daltons colocalizes with the cytoplasmic face of the nicotinic acetylcholine receptor (AChR) in approximately 1:1 stoichiometry. We show that this 43K protein can be phosphorylated in vitro by endogenous protein kinases present in AChR-rich membranes. The extent of 43K protein phosphorylation exceeds that of the subunits of the AChR, well-established substrates for enzymatic phosphorylation. We demonstrate that significant 43K phosphoprotein exists in vivo. The kinetics of phosphate incorporation mediated by endogenous kinases differed significantly from those of the AChR subunits, suggesting that different phosphorylation cascades are involved. Use of specific inhibitors of a variety of protein kinases indicated that endogenous cAMP-dependent protein kinase catalyzes phosphorylation of the 43K protein in vitro. All of the phosphate incorporated into 43K protein was accounted for by phosphoserine (0.65 mol/mol of 43K protein). Potential structural and functional consequences of 43K protein phosphorylation are discussed.