We establish a precision accelerator mass spectrometry (AMS) radiocarbon chronology for the Archaic period Tlacuachero shellmound (Chiapas, Mexico) within a Bayesian statistical framework. Carbonized twig samples were sequentially selected from well-defined stratigraphic contexts based on iterative improvements to a probabilistic chronological model. Analytical error for these measurements is +/- 15 to 20 C-14 yr. This greater precision and the absence of stratigraphic reversals eclipses previous C-14 work at the site. Based on this, we establish a chronological framework for a sequence of 3 clay floors dating to between 4930 and 4270 cal BP and determine that the bedded shell deposits that formed the mound accumulated rapidly during 2 episodes: a lower 2-m section below the floors that accumulated over a 0-150 cal yr period at 5050-4875 cal BP and, an upper 3.5-m section above the floors that accumulated over a 0-80 cal yr period at 4380-4230 cal BP.