Sucrose synthetase activity in endosperm extracts of seven shrunken(sh) mutants of spontaneous origin and three similar mutants due to the association of the controlling element Ds with the Sh locus is examined. A residual level of 3 to 5% as compared to the normal (Sh) endosperm is seen in all the mutants. The residual activity is similar to that of the Sh locus encoded endosperm sucrose synthetase by several criteria including an identical size of polypeptides and a similarity in antigenic properties. These two enzymes are, however, distinguishable by a slight difference in electrophoretic mobility in native gels and a difference in the relative abundance of enzyme molecules. The latter property is a reflection of a marked difference seen in the developmental profile of enzyme activity in the two genotypes. The earlier hypothesis (Chourey and Nelson 1976) that these two sucrose synthetases are encoded by two separate genes is strengthened by: (a) the presence of the residual enzyme in ash deletion mutant and (b) an electrophoretic demonstration of two proteins, corresponding to the major and minor sucrose synthetase proteins, in the wild type (Sh) genotype, The two sucrose synthetase genes seem to provide a model system in plants for studying the molecular basis of temporal specificity of genes.