The susceptibility of 2- and 4-month-old perennial ryegrass (Lolium perenne L.), orchardgrass (Dactylis glomerata L.), Kentucky bluegrass (Poa pratensis L.), red fescue (Festuca rubra L.) and highland bentgrass (Agrostis tenuis Sibth.) to glyphosate [N-(phosphonomethyl)glycine] was studied in the greenhouse. All 2-month-old grasses were killed by 0.28 kg/ha and higher rates of glyphosate. At 0.14 kg/ha, red fescue was moderately resistant, and bluegrass, orchardgrass, and perennial ryegrass were moderately susceptible. Bent-grass was very susceptible. When 4-month-old grasses were treated, bluegrass was as tolerant to 0.28 kg/ha of glyphosate as was red fescue. At this same rate, orchardgrass and perennial ryegrass were moderately susceptible, whereas bentgrass remained the most susceptible. Dosages lower than 0.28 kg/ha had little effect; whereas higher doages injured all five species. 14C-glyphosate was absorbed and translocated via both apoplast and symplast in 1-month-old red fescue, orchardgrass and perennial ryegrass seedlings. Comparatively, less radioactivity was transported to the untreated areas in red fescue than in orchardgrass and perennial ryegrass. Thus, the differential tolerances of these species to low rates of glyphosate may be explained, in part, by differential translocation of glyphosate.