We developed a novel system for online biomass measurement in shake flasks based on near 360° scattered light detection. The optosensoric components were combined with the established PreSens SFR Shake Flask Reader technology, so biomass, pO2 and pH can be monitored non-invasively through the flask bottom. First application of the prototype SFR vario on Escherichia coli K12 and Kluyveromyces marxianus culture resulted in highly reproducible online biomass data, with relative errors matching those of standard offline OD measurements.