Periphytic biomass has an important influence on the water quality of many shallow streams. The purpose of this paper is to synthesize the knowledge obtained on periphyton during the PIREN SEINE research program. Periphyton was sampled using chl a measurements by acetone extraction and oxygen measurements with microelectrodes. The experiments reveal the presence of an important fixed biomass ranging between 123 and 850 mgchl a m−2 and the mean gross production (photosynthesis) is shown to range between 180 and 315 mgC m−2 h−1. An independent approach was performed using the PROSE model, which simulates transport and biogeochemical processes in 22 km of the Grand Morin stream. A strong agreement between in situ measurements and the model results was obtained. The gross production obtained using PROSE is 220 mgC m−2 h−1 for the periphyton, which matches the experimental data. Although the net photosynthetic activity of the phytoplankton (0.84 gC gC−1 d−1) is higher than the periphytic one (0.33 gC gC−1 d−1), the absolute periphytic activity is greater since the mean biomass (3.4 gCm− 2) is 10 times higher than the phytoplanktonic one (0.3 gCm−2), due to the short residence time of the water body (1.5d).