Seed dormancy characteristics ofSpartina alterniflorawere delineated previously by Plyler and Carrick (American Journal of Botany, vol. 80, pp. 752–756, 1993). This study was undertaken in order to determine whether or not the dormancy characteristics ofS. patensare similar. As in the previous study, the site of a dormancy mechanism was determined by assessing the germinability of surgically altered dormant seeds. Likewise, the effects of three growth‐regulating substances (abscisic acid, fusicoccin, and gibberellic acid), prechilling, and light were studied by assessing the germinability of appropriately treated dormant seeds. Surgical injury to the endosperm, and to a lesser extent the scutellum, produced significant germination inS. patens, whereas only injury to the scutellum produced germination inS. alterniflora. Exogenously applied abscisic acid was highly effective in maintaining dormancy in injured seeds of both species. Gibberellic acid and fusicoccin were ineffective in promoting germination inS. patens, but fusicoccin was highly effective in breaking dormancy in 5.alterniflora.Prechilling was effective in breaking dormancy inS. patensbut not inS. alterniflora.InS. patens, treatments that broke dormancy were only successful when seeds were exposed to light during the germination period. It is concluded that the dissimilarities in the dormancy mechanisms may be manifestations of the different environmental adaptations these closely related species display.