Coastal habitats are increasingly being exposed to herbicide contamination from urban and agricultural catchments. Data on its toxicity on aquatic ecosystems, especially those based on sediment, are relatively scarce. This study aimed at investigating whether the susceptibility of an aquatic filter-feeding organism, the carpet-shell clam (Ruditapes decussatus) to the parasite Perkinsus olseni was influenced by the herbicide Roundup and its active ingredient glyphosate. The effect of Roundup and glyphosate on P. olseni in vitro proliferation was also evaluated and appeared to confirm the higher toxicity of Roundup when compared with technical grade glyphosate.