The use of formulations of the herbicide glyphosate in transgenic crops of the Pampa’s plains of Argentina has extensively increased, though there is scarce information of its impact on non-target vascular plants from agro-ecosystem related surface waters. The sensitivity of a local clone of the macrophyte Lemna gibba L. to glyphosate active principle and Roundup Max formulation was studied in standardized laboratory conditions. Phytotoxic effects, considering the aquatic route, at a concentration range of glyphosate between 0.5 and 80 mg L⁻¹ as active ingredient during 10 days of exposure were assessed on plant population growth, frond growth, shape and number, total chlorophyll content and colony architecture. Exposure to 1 mg L⁻¹ of glyphosate (an expected environmental concentration) affects all the studied assessment endpoints, except for population growth and chlorophyll content. Equivalent concentrations of this herbicide as the active ingredient or RoundupMax indicate higher phytotocity of the formulation. Exposed plants at concentrations of herbicide between 1 and 7.5 mg L⁻¹ exhibit after two days a recovery of the multiplication rate. Frond aggregation and longer stipe was detected between 1 and 15 mg L⁻¹ of glyphosate, determining more open colony architecture. At higher concentrations of the herbicide fronds break-up. Comparisons with literature data indicate a higher sensitivity of the L. gibba local clone with respect to L. minor and algal species, and also a similar response to the herbicide in field experiments with the same species.