Seasonal variations in phytoplankton biomass and production were studied in 1996 and 1997 in the newly flooded (1995) Sep Reservoir (France) in relation to various physico-chemical environmental factors. No trophic upsurge occurred in the reservoir as has been found in most of the newly flooded reservoirs. Phytoplankton composition changed drastically from 1996 to 1997, the latter year being characterized by a rarity of diatoms. Areal euphotic chlorophyll a concentration and phytoplankton carbon biomass were typical of mesotrophic lakes in 1996 and of somewhat oligotrophic environments in 1997. In contrast, the primary production (PP) rates were similar in both years, indicating a change in the factors controlling phytoplankton populations from 1996 to 1997. The low PP (compared with the algal biomass present) in 1996 seemed to be related to the instability of the water column and the Zmix:Zeu ratio, whose negative effects on the phytoplankton community were apparently more pronounced in 1996 than in 1997. Orthophosphate concentrations, and their contribution to total phosphorus, remained low in 1996 but increased with time in 1997. The temporal losses of orthophosphates from June to September 1996 accounted for only 42% of the measured PP, indicating that PP in 1996 was also strongly affected by P availability and probably took place due to other P sources such as regenerated nutrients.