Abstract: A method for monitoring the formation of biofilm on surfaces which are exposed to a flowpath of liquid essentially containing water includes flowing a liquid along the flowpath only once without being circulated through a tube section, through and along a series of sample tube pieces removably placed within the tube section, successively removing the sample tube pieces without replacement each time after a period of flow elapses and testing the sample pieces removed to determine the biofilm formation thereon. The tube section is arranged essentially vertically, and the flow through the tube section is downwardly directed, the uppermost sample tube pieces being removed in succession after each time period of flow exposure.