ABSTRACT: In symbioses between invertebrates and microalgae, host tissues are exposed to increased levels of photosynthetically produced oxygen. The biochemical consequences of symbioses have been poorly investigated in Mediterranean species, but ageneral increase in antioxidant defences has been recently reported in the demosponge Petrosia ficiformis as an adaptive response to the cyanobacterium Aphanocapsa feldmanni. Since Mediterranean symbioses naturally experience marked seasonalvariations in symbiont content, light intensity and seawater temperature, the aim of this work was to investigate if these fluctuations modulate the prooxidant challenge to sponge tissues. Antioxidant efficiency was characterised on a monthly basis bycombining an analysis of the main antioxidants (superoxide dismutase, catalase, glutathione S-transferases, glutathione reductase, glutathione peroxidases) with measurements of the total oxyradical scavenging capacity (TOSC), thus achieving a moreholistic assessment of the capacity of sponge tissues to absorb different forms of reactive oxygen species. Symbiotic sponges showed significant seasonal changes in antioxidant efficiency, with more marked variations in tissues directly exposed tophotosynthetically produced reactive oxygen species. The greatest variations were observed during the summer months, with the highest seasonal values for some defences (i.e. catalase) and the lowest for others (i.e. glutathione peroxidases). The markedincrease in catalase and TOSC in summer suggests greater production of H₂O₂ in the symbioses during this period, supporting the hypothesis that seawater temperature can significantly modulate the prooxidant challenge inMediterranean symbioses. The results suggest that species with lower antioxidant efficiency may be less tolerant of conditions effecting oxidative damage; e.g. increases in temperature during the summer months.