In situ diel variations of extracted chlorophyllous pigments, beam attenuation by particles (c _p), and in vivo chlorophyll fluorescence (F _iv) were investigated during a 5-day time series in high-nutrient, low-chlorophyll waters of the equatorial Pacific (date line = 180°). Samples were taken hourly at 10 depths in the upper 100 m during the first 48 hours, then sampling frequency decreased to 3 hours. In the 30–70 m layer the integrated chlorophyll concentrations, c _p, and F _iv increased during the light period, but the minima and, especially, maxima were not fully synchronized. The lowest values of total chlorophyll a (Tchl a = chlorophyll a + divinyl-chlorophyll a) occurred around 5–6 hours, slightly (0–2 hours) before that of c _p and F _iv. Tchl a reached a maximum around 1500 hours ± 1 hour, clearly before c _p (1700 hours) and F _iv (1900 hours). In the 0–30 m layer, diel variations of the integrated chlorophyll concentrations, c _p, and F _iv were clearly out of phase. They showed a nocturnal increase in Tchl a, starting around midnight and peaking in early morning (0900 hours). In contrast, c _p increased only during the light period in the upper 30 m, and variations of F _iv were largely opposite to those of extracted Tchl a. Specific phytoplankton growth (μ₀) and grazing loss (g) rates were estimated from diel variations in the 30–70 m layer and compared to independent rate estimates from experimental incubations. These results are discussed in the context of physical processes and physiological responses of the cells to the daily photocycle.