Abstract Photoacoustic spectroscopy was evaluated as useful for the determination of the number of cells in a fermentation. The influence of scattering particles and gas bubbles on the measurement was investigated. The photoacoustic method is much less influenced by scattering particles than the turbidity method. A fermentation of cyanobacteria was used to compare the photoacoustic method with turbidity, fluorescence and cell counting. The number of cells is represented by the photoacoustic data especially in the exponential and stationary phase with greater accuracy than the turbidity. The necessary optical set-up at the fermenter vessel is much simpler. It requires no probes incorporated in the biological system. By choosing a specific light wavelength specific cell products can be monitored.