Surfactants used in herbicide formulations are generally considered inert with no toxic effects on animals. Polyethoxylated tallow amines (POEAs) are non-ionic surfactants used in many herbicide formulations to promote the penetration of the active matter into plant cuticles. The present study aimed to assess the toxicity of a POEA surfactant system, the Genamin T-200®, on two larval stages of the Pacific oyster, Crassostrea gigas. The embryotoxicity of Genamin T-200® was quantified after 36 hr of exposure, considering both arrested development and abnormalities in D-shaped larvae. The ability of pediveliger larvae to metamorphose was studied after 24 hr exposure to Genamin T-200®. According to the European toxicity classification, the present results suggest that Genamin T-200® could be considered very toxic to embryo larval development, with an EC₅₀ of 262 µg/l, and toxic to metamorphosis processes with an EC₅₀ of 3,027 µg/l. The high toxicity of glyphosate-based formulations compared to the active ingredient and its by-product appears to be due primarily to surfactants.