The transmissometer has been actively used to monitor the variability of particulate matter in the surface ocean. Attenuation due to water (cw) is constant at a given wavelength, so variability in the signal is due primarily to particles (beam cp). Most of the beam cp signal appears to originate from particles <20 μm, which, in the euphotic zone, are most likely to be microorganisms. However, how much of the beam cp (λ=660 nm) is attributable to any given organism category or the detrital component is poorly known. To answer this question, at least numerical abundances and optical (scattering) cross sections of each category are needed. During the two transect cruises (TT007 and TT011) of the Joint Global Ocean Flux Study (JGOFS) Equatorial Pacific (EqPac) program (12 ¿N--12 ¿S, 140 ¿W), particulate matter attenuation (beam cp) and abundances of four microorganism categories (heterotrophic bacteria, Prochlorococcus, Synechococcus, and small autotrophic eukaryotes) with their forward angle light scattering (FALS) information (flow cytometer) were simultaneously measured from the same water samples. The bulk scattering coefficients of each population and total scattering coefficients (bp) of these four picoplankton populations (bp(PICO)) were calculated and compared with beam cp. In the equatorial Pacific, heterotrophic bacteria and Prochlorococcus were the most significant contributors to the beam cp (16 and 7% of beam cp, respectively) via scattering, and autotrophic eukaryotes and Synechococcus were less important contributors (2 and 1%, respectively) in the equatorial Pacific. If absorption was also included, ~30% of the beam cp could be accounted for by these four populations in the euphotic zone, supporting the argument that a transmissometer at 660 nm is an effective tool in measuring the net biological processes in the open ocean. A rough estimate for the beam cp signal from nonliving detrital particles was also made in surface waters of the region. ¿ 1998 American Geophysical Union