Abstract: A method for non-invasive growth measurement of organisms in an apparatus for growing and harvesting organisms or substances derived from such organisms. The organisms are comprised in an aqueous medium in a vessel, the method comprising the steps of: a) Providing means for measuring the opacity of the aqueous medium in order to estimate a growth rate of the organisms; b) The means for measuring the opacity are arranged in the apparatus such that they are located at predetermined locations relative to the vessel; c) The means for measuring the opacity are calibrated to account for refraction, absorbance and reflection caused by the vessel at the predetermined location. Finally the method also allows for measuring the opacity of the aqueous medium at the predetermined location.