The endangered Caribbean brown pelican (Pelecanus occidentalis occidentalis) was studied in the Greater Puerto Rican Bank from September 1980 to July 1983. Pelican numbers in Puerto Rican coastal waters averaged 1996 annually. Winter populations in Puerto Rico were 25% to 30% larger than summer populations because of an influx of newly fledged young and post-breeding adults from the U.S. Virgin Islands. Estimated mean annual pelican population size in the Bank region is 2300. Yearlings, subadults, and non-breeding adults tended to remain in Puerto Rican waters until breeding readiness caused their return to breeding colonies. Roosting and nesting habitat was comprised mostly of mangrove in Puerto Rico and offshore cays in the U.S. Virgin Islands. Used sites of mangrove vegetation had a mean height of 6.5 m, diameter at breast height of 13.2 cm, and stem density of 3557/ha. Feeding occurred in shallow, nutrient-rich waters associated with mangroves, estuaries, closed impoundments, and offshore cays. No relationship between pelican numbers and food availability (catchability) at selected feeding localities was detected. Pelican diet was dominated by Jenkinsia lamprotaenis, Anchoa lyolepis, and Harengula spp. Tilapia mossambica was an important prey species at estuaries and closed impoundments. Feeding success was correlated with age and water turbidity. Breeding efforts peaked during fall with extended breeding seasons in all colonies except those in western and southwestern Puerto Rico. Mean young produced per nesting attempt was 1.7 (1980), 0.59 (1981), and 0.71 (1982). These rates are believed adequate to maintain a viable population but further monitoring is recommended. Environmental contaminants, disease, climate and habitat availability are not limiting factors to pelican populations. Food and its distribution in space and time probably is the chief proximate factor influencing and regulating numbers of Caribbean pelicans.