Trichodesmium, a major colonial cyanobacterial nitrogen fixer, forms large blooms in tropical oligotrophic oceans and enhances CO2 sequestration by the ocean due to its ability to fix dissolved dinitrogen, however, its detection by satellite has not yet been successful in the South Western Tropical Pacific. Here, an algorithm has been developed for discriminating radiance anomalies observed in SeaWiFS imagery over the summertime Tropical Pacific. Its validation used 70 in situ observations of Trichodesmium accumulations for the period 1997-2004. The fraction of pixels identified as Trichodesmium in the region 5°S-25°S 160°E-190°E is low (between 0.1 and 0.5%) but is about 100 times higher than previous algorithms indicate. The algorithm reproduces the observed seasonal and inter-annual variability of Trichodesmium blooms in the SWTP.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.