In embryo cell cultures in gel medium infected with the Sindbis virus, two types of plaques appear large G and small p plaques. The G plaques appear in 24 hours and grow until they attain a diameter of more than 20mm. The small plaques p appear after 48 hours but never attain a diameter exceeding 3mm. After three successive reinoculations stable sub-lines of the p type are obtained, while the G sub-lines always give rise to a predominance of G but a small, constant percentage of p plaques. The interpretation of this phenomenon is that a mutation is spontaneously occurring, giving rise to small- plaque mutants from clones of wild-type, large-plaque viruses.