When investigating or quantifying the interaction of organisms with the hydrodynamic environment, it is often necessary or desired to use surrogates instead of the prototype organisms. In order not to derogate the results, it is important to design surrogates to represent the essential properties of the prototype organism correctly. To do so, several aspects of organism behaviour and morphology have to be considered, which are rarely mentioned in studies that present work carried out with surrogates. This paper presents a guideline to the choice and design of surrogates and aims to offer support during the design phase of flume studies, particularly to researchers that (i) are not (yet) familiar with living organisms and their behaviour, or (ii) have little experience with the strengths and limitations of hydraulic facilities, in order to enhance the quality of collected data and provide meaningful results.