Plants were grown to maturity in an experimental garden from a composite sample of seed collected randomly from ten widely separated populations along the Mississippi coast of the tidal marsh rush, Juncus roemerianus. Progeny segregated into two plant types: hermaphrodites bearing perfect flowers and females bearing pistillate flowers. No plants with staminate flowers were produced, nor were there any exceptions to the above distribution of flower types on individual plants. Examination of 1,323 flowering plants revealed that 861 (65%) produced only perfect flowers and 462 (35%) produced pistillate flowers. Almost twice as many plants bearing perfect flowers were produced in comparison to those bearing pistillate ones. The reason for this ratio is not presently known. This study provides experimental evidence and proof that Juncus roemerianus is gynodioecious. Furthermore, gynodioecism in J. roemerianus makes it unique among rushes and rare among monocotyledoneous plants.