Recently, poor control of Echinochloa colona with glyphosate has been reported in no-till agriculture systems of the northern grain region (NGR) of Australia. Two experiments were conducted using 10 populations of E. colona selected from the NGR of Australia to understand differences in their growth behavior and resistance pattern. Growth studies revealed that these populations differed in plant height (53-70 cm plant-1), tiller production (30-52 tillers plant-1), leaf production (124-186 leaves plant-1) and seed head production (37-65 seed heads plant-1). Days taken to seed heads and shoot biomass in these populations ranged between 40-48 d and 21-27 g plant-1, respectively. Seed production in these populations ranged between 5380 and 10244 seeds plant-1; lowest for population B17/25 and highest for population B17/13. Correlation studies revealed that seed number plant-1 had a positive correlation with tiller number plant-1 (r = 0.73) and negative relation with days taken to seed head initiation (r = - 0.65). The glyphosate dose-response study showed a wide range of responses in these populations and the glyphosate dose required to kill 50% plants (LD50 values) was estimated between 161 to 2339 g a.e. glyphosate ha-1. LD50 values of populations B17/16, B 17/34 and B17/35 were 1086, 2339 and 1153 g ha-1, respectively, making them 6.7, 15.1 and 7.2-fold resistant to glyphosate compared with the susceptible population B17/37. Growth behavior and seed production potential in these populations had no correlation with the resistance index. These results suggest that some populations of E. colona are highly problematic; for example, population B17/34 was not only highly glyphosate-resistant, but also produced a high seed number (9300 seeds plant-1). This study demonstrated that there is a possibility of great risk with the increased use of glyphosate for managing E. colona in the NGR of Australia. The results warrant integrated weed management strategies and improved stewardship guidelines are required for managing glyphosate-resistant populations of E. colona and to restrict further movement of resistant populations to other regions of Australia.