The aim of our experiment was to examine the influence of glyphosate on photosynthesis in plants and to check the usefulness of Sinapis alba for monitoring water contamination by glyphosate. The experiment was carried out in a phytotron chamber. It was divided into two parts. The first one determined the effect of a glyphosate herbicide (Glyfos 360 SL) on chlorophyll “a” activity in white mustard (Sinapis alba) seedlings placed in water. The second part assessed the white mustard response to contamination (by glyphosate) of water originating from the first part of the study. Changes in chlorophyll “a” activity occurred within 24h exposure of the plants to the herbicide. In the second part of the research, inhibition of over 47% of white mustard root growth was found as a result of 3-day exposure to water in which the test plant seedlings had been placed, following 48h-hour treatment with glyphosate herbicide at 7.2 ml/l concentration. This indicates the high utility of white mustard (Sinapis alba) in the assessment of environmental contamination by glyphosate.