Seed vigor in sweet corn (Zea mays L.) was compared among cultivars with the triple recessive endosperm mutant gene combination amylose-extender (ae), dull (du) and waxy (wx), the shrunken-2 (sh2) gene, their sugary (su) counterparts and an open-pollinated cultivar of normal genotype. Dry weight was significantly lower for F2 kernels of the high-sugar genotypes ae du wx and sh2 than for their su counterparts or the normal cultivar. The endosperm:embryo dry weight ratio was also low in the high-sugar lines due primarily to their small endosperm. Seedling dry weight at 10 days was correlated with endosperm:embryo ratio. Comparisons were made among the cultivars for shoot, radicle, and seminal lateral root growth from intact seeds and from excised embryos on nutrient agar. The normal genotype showed superior seed vigor when evaluated by seedling growth from intact kernels, but not when embryos were grown on agar, suggesting that vigor in normal was due to large endosperms. Respiration rate (μl O2 uptake/kernel·hr) of the germinating seeds did not account for growth differences among the genotypes. Respiration at 24 hr after imbibition was negatively correlated with seedling dry weight at 10 days. Respiration at 48 and 72 hr showed no significant correlations with growth rates. Low seed vigor in high-sugar genotypes apparently was related to their small endosperms. The genotype of the embryo also was important in seedling vigor, but low vigor in high-sugar cultivars could not be attributed wholly to genetic inferiority of the embryo.