12 October 1995 Below is a description of the analysis (modified from DuRand, M.D. and R.J. Olson, in press, Contributions of phytoplankton light scattering and cell concentration changes to diel variations in beam attenuation in the equatorial Pacific from flow cytometric measurements of pico-, ultra-, and nanoplankton, Deep-Sea Research): Niskin bottles from the CTD rosette were sampled during three diel time series on the equator at 140 degrees W (TT008 #1, TT012 #1, and TT012 #2) and were analyzed on a flow cytometer to obtain phytoplankton concentration for cells < 20 um diameter. The eukaryotic phytoplankton were analyzed immediately on board the ship using an EPICS V flow cytometer modified to analyze 50 ml seawater samples at 5-10 ml min-1 (Olson et al., 1991, 1993). Samples were kept at room temperature in the dark until analysis; the surface samples were analyzed first (the upper 45 m samples were analyzed within an hour) and all depths were completed within two hours of collection. During each cruise the phytoplankton groups were sorted on the flow cytometer and examined microscopically. Typically, the most abundant population of eukaryotic phytoplankton consisted of 1-2 um-diameter coccoid cells (referred to here as the ultraphytoplankton). The population referred to here as the nanophytoplankton primarily consisted of 2-3 um diameter coccoid cells, but also included cells as large as 20 um. The picoplankton Synechococcus and Prochlorococcus were analyzed on shore from samples preserved with 0.125% glutaraldehyde and stored in liquid nitrogen (Vaulot et al., 1989, Olson et al., 1993), using a high sensitivity configuration of an EPICS 753 flow cytometer (Olson et al., 1993). Synechococcus were distinguished by their high phycoerythrin fluorescence and Prochlorococcus by their low forward light scatter and chlorophyll fluorescence. Olson, R.J., E.R. Zettler, S.W. Chisholm and J.A. Dusenberry. (1991) Advances in oceanography through flow cytometry, pp. 351-399 in S. Demers (ed.), Particle Analysis in Oceanography, NATO ASI Series G 27, Springer-Verlag, Berlin. Olson, R.J., E.R. Zettler and M.D. DuRand. (1993) Phytoplankton analysis using flow cytometry, pp. 175-186 in P.F. Kemp, B.F. Sherr, E.B. Sherr and J.J. Cole (eds.), Handbook of Methods in Aquatic Microbial Ecology, Lewis Publishers, Boca Raton. Vaulot, D., C. Courties and F. Partensky. (1989) A simple method to preserve oceanic phytoplankton for flow cytometric analyses. Cytometry, 10, 629-635.