Biomass, growth and species composition of siliceous phytoplankton were studied in the Equatorial Pacific during October 1994. Experiments were carried out in different nutrient conditions along the Equator. An oligotrophic area, with nitrate concentrations as low as 10 nM in the upper layer, was encountered in the western part of the transect (166°E–170°W). The concentration of biogenic silica varied from 10 nmol 1−1 in the surface layer up to 40 nmol 1−1 in the deep chlorophyll maximum located near the nutracline. Biogenic silica production, measured by the 32Si method, showed a similar vertical pattern in the nitrate-depleted water, and the mean assimilation rate for Si was 0.4±0.2 nmol l−1 h−1 (integrated mean value: 63 μmol m−2 h−1). In contrast, nitrate concentration ranged from 2 to 4 μM in the surface layer in the high-nutrient low-chlorophyll (HNLC) area located from 170 to 150°W and biogenic silica increased to 200 nmol 1−1. The Si assimilation rate was 1.7±1.0 nmol 1−1 h−1 (integrated mean value: 162 μmol m-Z h−1). In both areas, 80% of Si biomass was concentrated in larger cells ( > 10 μm). Scanning electron microscopy was used to estimate diatom numbers and cell surface areas. This latter parameter correlates well with biogenic silica and warrants a discussion of the contribution of different species to the total biogenic silica. The measured values for specific uptake rate of Si never reached the optimum uptake or growth rate deduced from environmental parameters and kinetic constants reported in the literature. In addition the mean growth rate (0.9±0.3 doubling per day) for the nitrate-depleted water does not differ from the mean value (0.8±0.2 doubling per day) in the HNLC area. Therefore it can be concluded that diatom growth is severely limited in both regions. This agrees well with the nutrient balance study. In the oligotrophic area, N supply appears to be the limiting factor in the upper layer. In the HNLC region, the results of this study are consistent with the hypothesis that diatom growth might be limited by a micro nutrient such as iron. Carbon production by diatoms was estimated to be 31 ± 4 mmol Cm−2 d−1, which is one-third of the total carbon production of the nutrient-enriched area.