Laboratory experiments were performed on the amphipod Gammarus aequicauda in order to evaluate the relationship between the seasonal sensitivity to a reference toxicant and seasonal lipid content, temperature and algal biomass. The organisms were collected in a clean site in the Mar Piccolo of Taranto (Ionian sea) from March 2003 to February 2004. The total lipid content of G. aequicauda varied from 5.5% in spring to a maximum of 19% in autumn, then decreased until reaching 9.8% of dry weight in winter. Triacyglycerols (TG) were the most abundant lipid class in G. aequicauda, except in spring, when there was a higher percentage of phospholipids (PL). Sterol levels were lower. The sensitivity of G. aequicauda during different seasons was significantly affected by total lipids (r=0.97; p<0.01), but no relationship was found with algal biomass (r=0.21; p>0.05) and temperature (r=0.29; p>0.05). The highest LC(50) values appeared with an increase of total lipids. This study will be used to improve the interpretation of ecotoxicological test results with G. aequicauda.