Current concerns about climate change emphasize the need for an accurate monitoring of physiological conditions in wild populations. Therefore, the aims of this work were to a) assess the response of the octocoral Veretillum cynomorium to thermal variation in natural populations during low tide, by quantifying several biochemical indicators of thermal and oxidative stress and b) evaluate the effect of seasonality in the results and the adequacy of the use of biochemical indicators of stress in field monitoring studies in octocorals. Sampling took place during spring (April) and summer (June). Heat shock protein (Hsp70) and ubiquitin (Ub) content, enzyme activities — superoxide dismutase (SOD), catalase (CAT), glutathione-S-transferase (GST), and lipid perox-idation (LPO) were assessed in rachis and peduncle tissues separately. The results showed significant seasonal fluctuations in the set of biomarkers tested. Differences were detected between spring and summer, with significant decreases in biomarker levels from April to June being a major observed trend. These results suggest that V. cynomorium is thermo-tolerant during summer low tide conditions. Seasonal variation seems to reflect a metabolic suppression strategy and/or may also be related to seasonal changes in food availability and reproductive status. Differences in activity between tissue types were also found significant for GST, LPO and Ub. Biomarker levels were correlated with total protein concentration, but not with wet body weight of the specimens. This study suggests that season influences the expression of biomarkers and must be taken into consideration in the preliminary stages of sampling design for climate change biomonitoring studies. In addition, the results suggest that this octocoral species is likely to survive in future challenging thermal conditions.