In 1995 a massive bloom of phytoplankton composed primarily of diatoms was created by adding small quantities of iron to the surface waters of the equatorial Pacific. As part of this experiment, several bio-optical parameters were measured using drifter instruments and shipboard measurements. The iron-induced bloom resulted in dramatic changes in the bio-optical properties of the study site, potentially resulting in light limitation of the phytoplankton at the peak of the bloom. The results also provide evidence for a shift in photosynthetic energy partitioning by the phytoplankton assemblage upon initial exposure to iron. It is unclear, however, whether the physiological changes associated with the bloom were purely a result of the iron addition, or were also influenced by changes in species composition. We hypothesize that iron-induced changes in bio-optical properties of equatorial Pacific waters could have far-reaching effects on the global climate system. ¿ American Geophysical Union 1996