The family of cyprinids comprises some of the most important fish species in global aquaculture. With increasing global fish production also increasing amounts of suitable and sustainably produced high-quality feeds other than fishmeal are needed. Duckweed (Spirodela polyrhiza) is a fast growing aquatic plant with the potential of high biomass and protein production in nutrient recycling systems. This study reports the effects on growth, feed conversion and proximate body composition of common carp (Cyprinus carpio L.) fry fed graded levels of differently processed duckweed (meals dried and fermented) each replacing 150, 300 or 450 g/kg of dietary fishmeal protein. Comparisons were made to a duckweed-free and fishmeal-based control, containing 400 g/kg of fishmeal, equivalent to 261.9 g/kg of fishmeal protein. Carp fed the highest inclusion rate of fermented duckweed showed a significantly reduced performance (growth and feed conversion rate) compared to all other groups. No differences regarding growth and feed conversion rate were found among all other groups. Whole body crude lipid content was generally lower and crude ash content generally higher in carp fed dried compared to those fed fermented duckweed. Whole body crude protein content was not influenced by treatments. Based on the results of this study, dried S. polyrhiza meal could be used to replace up to 450 g/kg of fishmeal protein, while fermented S. polyrhiza meal should only replace up to 300 g/kg fishmeal protein in diets for carp fry