Contributions of different microbial groups to absorption, backscattering, and marine reflectance (a(λ), bb(λ) and R(λ), respectively) were quantified during an El Ni¿o-Southern Oscillation cold phase in the equatorial Pacific during the Etude du Broutage en Zone Equatoriale cruise on board the R/V L'Atalante. In situ data were collected at every degree of latitude from 8¿S to 8¿N, 180¿ (26 October to 13 November 1996), and satellite reflectances were available from POLDER-ADEOS for the 1--10 November 1996 decade. Bulk absorption and backscattering coefficients were estimated at 440 nm for the major microbial groups enumerated in the upper surface layer (heterotrophic bacteria, Prochlorococcus, Synechococcus, and <20-¿m eukaryotic algae). Total absorption and backscattering coefficients were retrieved from space by a new inverse method. The observed ecosystem was typical of a well-developed equatorial upwelling, with maximal values of 0.4 mg m-3 for Tchl a, 0.026 m-1 for ap(440), 0.023 m-1 for aphy(440), and a low in situ adet (<14% of ap). Prochlorococcus and nanoeukaryotic algae (3.4-¿m mean diameter) were the dominant absorbers (97%), contributing about equally to aphy. The retrieved total absorption coefficient, asat(440), from POLDER (maximum of 0.03 m-1) was higher than ap(440), as it included absorption by CDOM (estimated to be 15% of ap(440) + aw(440), where aw = absorption by pure water). Heterotrophic bacteria were the dominant contributors (73%) to total simulated microbial backscattering, bbmic (maximum = 3.7 ¿ 10-4 m-1), but bbmic was negligible compared to the inverted total backscattering by particles, bbp (2.7 ¿ 10-3 m-1), indicating that unidentified small nonliving particles contributed most to the satellite signal.