During most of the growing season of 1994, pigment content, as determined by HPLC analysis of algal sample extracts, was followed in the River Meuse (Belgium) potamoplankton. The concentration of some algal pigments (chlorophylls a and b, fucoxanthin, lutein, echinenone and alloxanthin) was related to biomass estimates of total phytoplankton and of major taxonomic components (diatoms, green algae, cyanobacteria and cryptomonads). Highly significant linear regressions were obtained for chlorophyll a-total biomass, fucoxanthin-diatoms, lutein-green algae, chlorophyll b-green algae. However, no relationship was found for cyanobacteria or cryptomonads and their specific pigments, which may be attributed to poor accuracy of biomass estimates for these nondominant algae. In conclusion, the good relationship found for dominant algae and their specific pigments confirms the value of pigments as quantitative markers of phytoplankton, as detected in other marine and freshwater environments.