Studies involving 32P labeling and wet ashing of isolated dynein reveal that isolated dynein contains approximately 6 mol of phosphate predominantly distributed over four polypeptides of molecular masses of 78, 76, 47, and 23 kDa. Dynein must, therefore, be phosphorylated to at least this extent in vivo. The catalytic subunit of cAMP-dependent protein kinase and an axonemal cAMP-dependent protein kinase contaminating the dynein preparation can further phosphorylate dynein in vitro. Each kinase can place up to 0.5 mol of phosphate on native dynein polypeptides of molecular masses of 78 and 34 kDa. Removal of two of the phosphates on isolated dynein by either acid or alkaline phosphatase results in a 28% decrease in the specific activity of dynein in the presence or absence of microtubules. Selective attenuation of the microtubule-activated ATPase, but not the uncoupled free dynein ATPase, would be indicative of a regulatory function of the phosphates. The in vivo regulation of the dynein ATPase by the two phosphates accessible to acid or alkaline phosphatase is therefore subject to question. Other phosphates on dynein must be examined for their effect on the microtubule-dynein cross-bridge cycle and motility before phosphorylation can definitively be established as a mode of dynein regulation.