Microplastics and pesticides are found in coastal waters around the world today. Their high uses and production make them emerging pollutions. These pollutants are likely to have an impact on the physiology of marine organisms, such as the oyster (Crassostrea gigas). In this present study, we monitored the valve activity of oysters, C.gigas, exposed for 24 days, following a 7-day acclimatization period, to polyethylene (HDPE 20-25 µm) microplastics (MPs, 10 µg MP/L) alone or combined with the herbicide, chlortoluron (30 µg/L). The valve activity of juvenile oysters (12-month-old) was determined using four parameters. HFNI technology (high frequency and non-invasive valvometer) allows us to study the daily growth of the oyster shell thanks to the electrodes attached to the two oyster valves. In addition, the HFNI system makes it possible to measure the number of micro-closures (stress factor in bivalves), the Valve, Opening Duration (VOD) and Opening Amplitude (VOA). We observed a significantly higher frequency of micro-closures in oysters exposed only to MPs after six days and until the end of the exposure. No significant effect was observed on the VOA for oysters exposed to MPs alone. VOD was significantly reduced for oysters exposed only to MPs. For the cocktail conditions (MPs + chlortoluron), a significant effect was observed with the increase in the number of micro-closures and a decrease in the VOA. Finally, the growth of the oyster shell was slowed down in the presence of MPs alone with a significantly reduction of 50.4 % and for the cocktail condition with 42.7 % of growth reduction. This study provides evidence for the impact of MPs on valve behaviour and oyster shell growth, using high but realistic MPs concentrations. Moreover, additional analyses will be carried out on toxicity biomarkers, oxidative stress, metabolites and energy reserves potentially impacted by the MP exposure.