Chlorophyll has long functioned as the prominent field metric for phytoplankton biomass, but its variability can be strongly influenced by (even dominated by) physiological shifts in intracellular pigmentation in response to changing growth conditions (light, nutrients, temperature). The particulate beam attenuation coefficient (cp) may offer an alternative optical measure of phytoplankton biomass that is readily assessed in situ and relatively insensitive to changes in intracellular pigment content. Unlike chlorophyll, however, cp is not uniquely associated with phytoplankton and varies as well with changes in inorganic, detrital, and heterotrophic particles. In open ocean environments, particles in the size range of ∼0.5 to 20 μm (i.e., within the phytoplankton size domain) dominate cp. Multiple field studies have indicated that the ratio of cp to chlorophyll (i.e., c*p) registers first-order changes in algal physiology, suggesting that cp covaries with phytoplankton carbon biomass. Here we use approximately 10,000 measurements of cp and fluorescence-based chlorophyll estimates (ChlFl) to evaluate the correspondence between these two phytoplankton biomass proxies. Our study focuses on a region of the eastern equatorial Pacific where mixed layer growth conditions are relatively homogeneous, thereby constraining phytoplankton chlorophyll:carbon ratios and allowing chlorophyll to function as a reliable measure of phytoplankton biomass. Over our 6600 km transect, cp was exceptionally well correlated with ChlFl (r2 = 0.93). Our results contribute additional support for cp as a viable index of phytoplankton carbon biomass in the open ocean.