An analysis of five Holocene sediment cores from Belize atoll lagoons has revealed a possible mass mortality of the Caribbean sea urchins Diadema antillarum and Echinometra sp. during 6400–6100, 4000, and 1300 yr BP. Event layers were identified by quantifying echinoid fragments in Holocene sediments. Peaks of echinoid abundance were dated radiometrically. There are two explanations for the fact that echinoderm-rich layers cannot be correlated among the three atoll lagoons. First, die out was not as widespread as the 1983–84 event that led to mass mortality of Diadema in the Caribbean. Possibly, local environmental transitions in lagoonal circulation patterns and changes in temperature and precipitation in the study area were responsible for echinoid mass mortality in the Holocene of Belize. Second, echinoid mass mortality is not necessarily implied in the composition of contemporaneous reef sediments. Comparable studies on Holocene corals in the area also suggest that turnover events have repeatedly occurred during the past several thousand years, however, recent events were unprecedented in their significance. Potential limitations of this study refer to taphonomic bias as imposed by callianassid shrimp burrowing.