St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] is a warm-season turfgrass with medium to high shade tolerance, a valuable trait for use in lawns, particularly in smaller residential landscapes and other green spaces where trees are dominant. However, their long internodes and very coarse leaf and stolon texture are undesirable in home lawns and public spaces. Gamma (g) irradiation has been used to induce useful variations for various morphological traits in turfgrass breeding. The objective of the present study was to induce dwarf and semidwarf phenotypes from two St. Augustine germplasms, S1 and S2, irradiated with 50, 100, 150, or 300 Gy and 20, 40, 80 or 120 Gy respectively, using a Co-60 source. The lethal does (LD50) for S1 and S2 genotypes were 95 and 103 Gy, respectively. The linear reduction of survival rate with increasing gamma rays was highly correlated (r(2) = 0.96 and 0.87). The 15 morphological mutants (0.35% of the irradiated plants) with a semidwarfed growth habits were identified among 4301 node cuttings. The mutant lines exhibited up to 40% shorter plant height and finer leaf and stolon texture than parental genotypes. Mutant lines developed in this study may offer improved St. Augustinegrass cultivars for landscaping.