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As part of the study of the eutrophic lake Aydat (Massif Central, France), the energy charge (CE = ATP + 1/2 ADP/ATP + ADP + AMP) and photosynthetic activity of size fractionated phytoplankton (0,2-160 μn; 0,2-45 μm and 12-160 μm) were assessed from April to November (1985). Weekly samples were gathered from several depths in both the euphotic and dysphotic zones. The fraction 0,2-45 μm showed elevated primary production and weak energy charge percentages confirming the generally accepted idea about the increase of metabolical cell processes with lower cell size. We have also looked for the ecophysiological significance of the energy charge through its seasonal coupling with primary production. The highly significant negative correlations obtained between CE and primary production for the three fractions indicate that the pool of adenine nucleotides evolves towards forms less charged with a high energy phosphate bond. However, some exceptions to this rule were noticed in April and June (both CE and photosynthetic activity were high). This observation derived from in situ studies showed the difficulties encountered in natural environments when we try to check the ecophysiological significance of the energy charge.