Technical Abstract: Italian ryegrass tolerant to glyphosate is becoming a major weed problem in glyphosate-resistant crops. The effects of temperature, light, pH, salt and osmotic stress, shikimic acid, and planting depth on germination of glyphosate-tolerant (T) and susceptible (S) Italian ryegrass populations were studied. Overall, germination of both Italian ryegrasses was highest at 13 C and decreased when temperature increased to 20 or 27 C under both light and dark conditions. In S Italian ryegrass population, light stimulated germination (57%) compared to darkness (41%) at 13 C, but light had no effect on germination at 20 and 27 C. In T Italian ryegrass population, light had no effect on germination at 13 C while light inhibited germination at 20 and 27 C. The T Italian ryegrass had higher germination (69 to 87%) compared to the S Italian ryegrass population (37 to 57%) at a pH 4 to 7 range. Seed germination decreased as NaCl concentration increased from 20 to 160 mM and osmotic potential increased from 0 (distilled water) to –0.8 MPa in both the T and S Italian ryegrass populations. Germination of T and S Italian ryegrass populations reduced from 76% to 25% and 67% to 12%, respectively, as shikimic acid concentration increased from 0 to 16 mM. Seedling emergence was highest from seed placed on the soil surface. Seedling emergence was less than 7% from seed planted at 0.5 cm depth and none emerged from seed planted below 2.5 cm depth in both populations. These results suggest that both the T and S Italian ryegrass have the ability to germinate under a broad range of environmental conditions. There were also some differences in germination response between the T and S Italian ryegrass populations. The T Italian ryegrass population germinated better than the S Italian ryegrass population across all the environmental factors studied.