The Late Holocene sedimentological and palaeoecological evolution of the Grosseto alluvial plain (Tuscany, Italy) was studied using approximately 120 lithological borehole descriptions, palaeoecological records and five <sup>14</sup> C dates. During the Holocene, the Ombrone River migrated laterally, but did not avulse. New radiocarbon dates allow average sedimentation rates to be calculated in the order of approximately 0.9 mm/yr before 2800 cal yr BP and approximately 1.4 mm/yr between 2800 cal yr BP and the present. This is considerably lower than hitherto presumed. The new <sup>14</sup> C ages can be related to the relative sea-level curve for the Tyrrhenian Sea. The results of macro- and micro-fossil analysis suggest increasing human influence since approximately 2800 cal yr BP and an environmental change from brackish-marine to freshwater conditions. Recent marine influence led to saltmarsh conditions.