In order to determine the concentration and distribution of the phytoplankton biomass in waters of the islands of Old Providence and Santa Catalina, Colombian Caribbean a survey with 47 superficial samples, 12 in the whole water column, was carried out in 2005. Physicochemical parameters varied through the year; temperature fluctuated between 25 and 30 °C, salinity between 28 and 39, and pH between 8.30 and 9.65. Chlorophyll a and phaeopigment a concentrations were low, a characteristic of oligotrophic oceanic areas; chlorophyll a presented an average of 0.198±0.017 μg/L and 0.693±0.149 μg/L for dry and rainy seasons, while the phaeopigment a had a value below 0.01 μg/L in both seasons. It was found that the phytoplanktonic community is young and with an elevated renovation rate.