ABSTRACT: The growth rates and buoyancy properties of 3 oceanic diatoms in the genus Rhizosolenia were examined at light levels from 8 to 211 umol quanta m^-2 s^-1. Maximum growth rates ranged from 0.37 to 0.78 divisions d^-1with saturation occurring between 29 and 164 umol quanta m^-2 s^-1. Severe growth rate depressions were noted in R. acuminata and R. formosa at irradiance levels above 50 to 155 umol quanta m^-2 s^-1.In all 3 species the percentage of positively buoyant cells was inversely related to light intensity. In R. formosa both growth rate and tolerance to high light levels decreased substantially as cell size decreased. Batch culture C:chlorophyllratios (130 to 261) replicated values found in field Ethmodiscus and Rhizosolenia mats, and suggest that the elevated C:chlorophyll ratios found in buoyant, oceanic phytoplankton are typical of healthy cells. Calculations suggest thatcarbohydrate ballasting can account for buoyancy changes and that these reserves are adequate to support dark NO₃^- uptake. Under steady-state conditions in situ, the observed growth and buoyancy properties would lead tosubsurface population maxima in all 3 species. However, the dynamic light-related buoyancy changes probably occur on a shorter time scale than these batch culture experiments. These results indicate that vertical migration is a property basic to thesediatoms life history strategy, and, like multispecies Rhizosolenia mats, solitary Rhizosolenia chains transport new nitrogen to the euphotic zone in oligotrophic seas.