AbstractGestal, C., Guerra, A., and Pascual, S. 2007. Aggregata octopiana (Protista: Apicomplexa): a dangerous pathogen during commercial Octopus vulgaris ongrowing. – ICES Journal of Marine Science, 64. The haemolymph parameters for Octopus vulgaris and the condition index at molecular level were analysed using RNA/DNA and RNA/protein ratios on animals reared in floating cage systems and naturally infected by the coccidian Aggregata octopiana. Statistical analysis showed that as A. octopiana infection increased, there was a decrease in most inorganic elements and/or in haemocyte concentrations in cellular and/or plasma fractions. Also, the protein concentrations in the haemolymph plasma fraction dropped significantly as the coccidian infection increased. Results suggest that the severity of the infection by A. octopiana is a limiting factor during commercial ongrowing of the common octopus, at least in floating cage systems. Control measurements are needed in aquaculture to avoid this environmental stressor.