Polypeptides preferentially labeled in the G2 phase of the synchronous nuclear replication cycle of Physarum macroplasmodia were compared in electrophoretic mobility and peptide map with the tubulins enriched from Physarum myxamoebae. One major and one minor fluorographic species match the myxamoebal alpha and beta chains, respectively. Thus, tubulins are among the proteins of Physarum selectively synthesized before nuclear division. A third species P, prominently labeled in premitotic plasmodia, is distinct from the two myxamoebal tubulins even though it co-polymerizes with microtubules. The nature of P remains unknown. Two rat monoclonal antibodies directed against yeast tubulin were found to bind selectively to the alpha tubulin of porcine brain. These served to confirm the assignment of the 50,000-dalton Physarum myxamoebal tubulin as an alpha-like polypeptide.