The biomass and production of the phytoplankton in a relatively unpolluted reach of the River Meuse were followed through 1983 and 1984. Chlorophyll a varied from 0.2 to about 120 mg/cu m, and production ranged between 0.05 and 5.78 g C/sq m/day. The mean photosynthetic quotient was 1.25. The parameters of the light-photosynthesis relationship (P sub opt = rate of light-saturated photosynthesis, and l sub k = onset of light saturation of photosynthesis) were calculated and related to the variations of temperature and light in the water column. A simple model allowed calculations of the annual production, which was estimated to be 494 g C/sq m/yr in 1983 and 547 g C/sq m/yr in 1984. A model was developed that explains the relationship between phytoplankton development and discharge: this model shows how the effect of discharge can be described by a ' dilution rate ' of the plankton growing in the river water. ; SCOPUS: ar.j ; info:eu-repo/semantics/published