IT is now widely believed that a large impact occurred on the Earth at the end of the Cretaceous period, and that the buried Chicxulub structure in Yucatan, Mexico, is the resulting crater(24). Knowledge of the size and internal structure of the Chicxulub crater is necessary for quantifying the effects of the impact on the Cretaceous environment. Although much information bearing on the crater’s structure is available, diameter estimates range from 170 to 300 km (refs 1-7), corresponding to an order of magnitude variation in impact energy. Here we show the diameter of the crater to be similar to 180 km by examining the horizontal gradient of the Bouguer gravity anomaly over the structure. This size is confirmed by the distribution of karst features in the Yucatan region (mainly water-filled sinkholes, known as cenotes). The coincidence of cenotes and peripheral gravity-gradient maxima suggests that cenote formation is closely related to the presence of slump faults near the crater rim.