Rheological behaviour of culture broth stands as a fundamental parameter in bioprocess performances because it affects simultaneously the heat and mass transfer as well as the flow pattern. On-line measurements of rheological behaviour are hardly compatible with the operating condition with respect to accurate and stringent conditions imposed by cell culture strategy. Our scientific and technical objectives are (i) to develop and identify an experimental device enabling on-line rheometry and (ii) to discuss and compare on-line and off-line measurements. In this aim, a bioreactor was equipped with a derivation loop including a specific on-line rheometric device as well as additional physical and biological measurements (specific density, mass flow rate, electrical conductivity, pH, pO 2 and temperature) during microbial cell cultures. In a first time, friction curves of calibrated ducts were established with New-tonian and non-Newtonian shear-thinning fluids. In a second time, axenic cultures with two microorganisms (bac-teria and yeast exhibiting different sizes) were investigated in pure oxidative culture in order to produce biomass under high cell concentrations: ~ 40 to 110 g CDW /l for E. coli (bacteria) and ~ 75 to 105 g CDW /l for Y. lipolytica (yeast). Cell broths exhibited Newtonian behaviour for E. coliand shear-thinning behaviour for Y. lipolytica, which were both depen-dant on biomass concentration. On-line and off-line rheological measurements are consistent for E. coli and Y. lipoly-tica, but significantly differed. On-line estimated viscosity appears higher than off-line apparent viscosity. Several assumptions in relation with microorganism physiology and metabolism (size, morphology, surface properties, concentration , biological activity) could be formulated in agreement with scientific literature. On-line rheology brings new insight to investigate complex interaction between physical and biological phenomena.