The quantitative trace determination of glyphosate and its major metabolite, aminophosphonic acid (AMPA) in natural waters was achieved by means of ion-exchange chromatography. Fifty ml of natural water sample was preconcentrated by a two-step procedure: first the sample was percolated through a polymeric cartridge, LiChrolut EN, then through an anion-exchange column mechanism, and finally analyzed by ion-exchange chromatography followed by post-column reaction coupled to a fluorimetric detector. Linear calibration graphs were obtained between 5 and 200 micrograms l-1. Limits of detection ranged from 2 micrograms l-1 of glyphosate and 4 micrograms l-1 of AMPA. A study of the degradation of glyphosate in environmental waters under characteristic conditions was carried out to figure out the main degradation pathways of this compound. Half-lives of glyphosate varied from 60 h for ground water samples exposed to sunlight to 770 h for those stored under dark conditions.