Cytoplasmic dynein is a minus-end-directed, microtubule-dependent motor composed of two heavy chains (approximately 530 kDa), three intermediate chains (approximately 74 kDa), and a family of approximately 52-61 kDa light chains. Although the approximately 530 kDa subunit contains the motor and microtubule binding domains of the complex, the functions of the smaller subunits are not known. Using two-dimensional gel electrophoresis and proteolytic mapping, we show here that the light chains are composed of two major families, a higher M(r) family (58, 59, 61 kDa; dynein light chain group A [DLC-A]) and lower M(r) family (52, 53, 55, 56 kDa; dynein light chain group B [DLC-B]). Dissociation of the cytoplasmic dynein complex with potassium iodide reveals that all light chain polypeptides are tightly associated with the approximately 530 kDa heavy chain, whereas the approximately 74 kDa intermediate chain polypeptides are more readily extracted. Treatment with alkaline phosphatase alters the mobility of four of the light chain polypeptides, indicating that these subunits are phosphorylated. Sequencing of a cDNA clone encoding one member of the DLC-A family reveals a predicted globular structure that is not homologous to any known protein but does contain numerous potential phosphorylation sites and a consensus nucleotide-binding motif.