Trends in weed richness responses to widespread use of glyphosate tolerant soybean in Argentina are presented. An experiment was carried out since 1997 to analyse weed community changes associated with the continuous and exclusive use of glyphosate. Sampling results showed that weed richness in soybean decreased or remained stable early in the season before glyphosate application but increased at harvest. A survey of crop advisers indicated that 37 species were of increasing, 18 species of decreasing importance. It is suggested that successional changes are taking place in the cropping systems in response to changes in weed control practices..