A recombinant protein for the S-layer protein of Aeromonas hydrophila was produced and its ability to protect common carp Cyprinus carpio L. against six virulent isolates of A. hydrophila was assessed. A group of 120 carp (30–40 g) were vaccinated intra-peritoneally with 0.1 ml of adjuvanted vaccine (30μg protein per fish). Another group of 120 carp were injected with 0.1 ml of PBS-adjuvant mixture to serve as controls. Twenty fish from each group were challenged with each one of six virulent isolates of A. hydrophila 35 days post-vaccination. The fish were maintained in 12 separate tanks before terminating the experiment at 16 days post-challenge. The relative percentage survival (RPS) for the six isolates of A. hydrophila ranged from 56 to 87%. The difference in survival rate of fish challenged with four of the isolates was statistically significant in vaccinated fish compared to control fish, when analysed using a Chi-square test. The results of the study suggest that the recombinant S-layer protein of A. hydrophila could be useful as a vaccine antigen to protect fish against different isolates of this pathogenic bacterium. Vaccine 28 (2010) 3540–3547. Saravanane Poobalane, Kim D. Thompson, László Ardó, Noel Verjan, Hyun-Ja Han,Galina Jeney, Ikuo Hirono, Takashi Aoki, Alexandra Adams. Institute of Aquaculture, University of Stirling, Stirling, FK9 4LA, UK Research Institute for Fisheries, Aquaculture and Irrigation, Anna liget 8, H-5540 Szarvas, Hungary Laboratory of Genome Science, Graduate School of Marine Science and Technology, Tokyo University of Marine Science and Technology, Konan 4-5-7, Minato, Tokyo 108-8477, Japan.