No formal population studies of American Crocodiles (Crocodylus acutus) and Spectacled Caimans (Caiman crocodilus) of the Northern Zone of Costa Rica were conducted prior to 2019. We estimated the status of these populations during 2019–2020, specifically in the rivers considered representative of the area (Niño River, Frio River, San Carlos River, and Sarapiquí River). We performed two iterations of night count to estimate relative abundance and captured individuals to determine sex and measure body size. We observed C. acutus only in the San Carlos and Sarapiquí Rivers and C. crocodilus only in the Niño and Frio Rivers. We estimated a total population of 243 C. acutus, with a relative abundance of 1.84 individuals/km and a sex ratio of 1.4 males: 1 female. For C. crocodilus, we estimated a total population of 321 individuals, with a relative abundance of 11.88 individuals/km and a sex ratio of 2 males: 1 female. Population dynamics of these species are similar in the North Zone and in their ecologically and spatially linked populations in Costa Rica’s Central and North Caribbean zones.

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