Knowledge of demographic aspects of caimans and crocodiles is a very important issue for their conservation. A study was carried out in the Sierpe River, in the South Pacific of Costa Rica, from July 2004 through March 2005, comprising both rainy and dry seasons. Nocturnal visual surveys were conducted using shining flashlights; all the individuals were captured, measured and sexed, using a four ranges classification. During the eight surveys, a total of 58 individuals were captured, 65.51% belonging to class II, and 31.03% to class III. Important human explotation has been documented for this population. Differences in male/female ratio and the capture numbers in the study area can be explained by the migration of individuals.