Fatty acids are essential elements for the structure of biological membranes and for the storage of metabolic energy. They are used as a source of energy by metabolism at each trophic level, making fatty acids biochemically and physiologically important compounds (Neves et al. 2015). In the trophic chain, many fatty acids are only synthesized by microalgae and bacteria before being transferred via herbivorous invertebrates to fish and ultimately to humans (Arts et al. 2001). For example, highly unsaturated fatty acids (HUFA) such as eicosapentaenoic acid (EPA; C20:5n3), cannot be synthesized de novo or in insufficient proportions by animals (Saito and Aono 2014). That is why fatty acid analysis is commonly used to study trophic interactions in food chains. Generally, microalgae with a high proportion of EPA, such as diatoms, are an excellent source of food for animals but the concentrations of these different fatty acids can vary according to different environmental parameters including temperature, and may be affected by pesticide exposure (Brett et al. 2006, Robert et al. 2007, Burns et al. 2011, Li et al. 2014, Filimonova et al. 2016). The aim of this study is to investigate the impact of two herbicides combined to a 3°C-temperature increase on diatom's fatty acids and photosynthesis. To address this issue, two model freshwater diatoms (Gomphonema gracile and Nitzschia palea) were exposed to two herbicides, with two different cellular targets, at high concentration (diuron and s-metolachlor, C=10 µg/L) and two temperatures (T1=17,5 °C and T2=20,5°C). After a 1-week exposure, the analysis of photosynthesis was carried out by PhytoPAM measurements, and fatty acid composition of diatom cultures was determined by gas chromatography. For the control samples, the photosynthesis increased with the temperature for the both species mainly for Gomphonema gracile. Concerning the fatty acids composition of the both species, saturated and monounsaturated fatty acids decreased with temperature while polyunsaturated fatty acids increased. The 3°C increase in temperature enhanced the toxicity of diuron towards the photosynthesis of both species, while it reduced the effect of S-metolachlor for Gomphonema gracile. Fatty acids analysis are still ongoing. Our results highlight contrasting effects of a moderate increase of temperature on the toxicity of pesticides towards diatoms, at the physiological (photosynthesis) and structural (fatty acid content) levels.