Physarum myxamoebae can be reversibly induced to become flagellates. Physarum flagellates contain a new form of tubulin, alpha 3, that is not found in nonflagellated cells. Evidence is presented that suggests that alpha 3 tubulin arises through posttranslational modification of a preexisting alpha tubulin. Pulse-chase experiments showed that labeled alpha 3 tubulin could be detected when flagellates formed after a chase. RNA was isolated from myxamoebae at different times after induction of flagellum formation. When this RNA was translated in vitro, the resulting products contained no alpha 3 tubulin, also consistent with alpha 3 being made by posttranslational modification. Levels of alpha and beta tubulin RNA increased with the proportion of flagellates in the culture. These elevated tubulin RNA levels declined after the number of flagellates in the population achieved plateau values.