ABSTRACTEthmodiscus rex(Rattray) Wiseman and Hendey cells from near surface net tows in the Southwest Atlantic Ocean and Caribbean Sea were examined for chemical composition, internal nutrient pool concentrations, and oxygen evolution characteristics. Elemental ratios indicated nitrogen limitation with C:N:P ratios of 125:9:1 (atoms), and carbon: chlorophyll (chl) ratios of 129:1 (weight). However, internal nitrate pools (1.4–27.1 mM) suggested that cells were notN‐limited. Intracellular NO3−accounted for up to 54% (range = 3–54%) of the total N quota in some samples. Photosynthetic parameters were consistent with a high‐light‐adapted population and suggested an instantaneous maximum chl‐specific photosynthetic rate (PBmax) of 4.8–12.4 nmol O2·μg chl−1· h−1. Respiration rates varied ten‐fold and were inversely related to PBmaxEthmodiscuschemical composition and buoyancy characteristics are similar to vertically migratingRhizosoleniamats and the non‐motile dinoflagellatePyrocystis noctilucaMurray (Schuett). The presence of internal NO3−pools inEthmodiscussuggests that this genus is also vertically migrating to exploit sub‐surface nitrogen pools. Such behavior may be widespread in large, non‐motile oceanic phytoplankton. Based on ascent rate data, chemical composition, and photosynthetic rates, we estimate that the entire division–migration cycle forEthm