Temperature-dependent growth restriction was studied with the measles vaccine strains licensed in Japan in comparison with their parental wild strains. Plaquing efficiency was compared at various temperatures from 35 to 40 C. AIK-C strain derived from Edmonston wild strain was temperature-sensitive with the 39C shutoff temperature. No significant restriction of growth was found for other vaccine strains, i.e., Schwarz, FF-8, and CAM-70, and for their parental wild strains, i.e., Edmonston and Tanabe. A paradoxical feature was found for FF-8 strain; in spite of undiminished plaquing efficiency at 40C, the growth in the fluid medium was markedly depressed at 39C or above.