Oral vaccination is a relevant alternative for fish immunisation in intensive culture. However, its effectiveness is limited by possible vaccine degradation in the fish digestive system. The purpose of this work was to obtain stable biocompatible alginate microparticles entrapping inactive Aeromonas hydrophila cells for fish oral immunisation. The particles were prepared through an emulsion-based methodology, employing different vegetable oils containing the surfactant Span 80, varying the ratio between aqueous alginate and oil phases, the stirring rate and the initial concentration of A. hydrophila. Microparticles stable under gastrointestinal conditions presenting mean diameters lower than 50 mu m were obtained with 1:9 (v/v) of aqueous phase containing 3.5% alginate (w/v) in relation to the oil phase (corn oil with 0.2% v/v Span 80) under stirring at 2000 rpm, with bacterium encapsulation efficiencies up to 100%. (c) 2005 Elsevier Ltd. All rights reserved.413638643