Walsh, K. D., Soltani, N., Hooker, D. C., Nurse, R. E. and Sikkema, P. H. 2015. Biologically effective rate of sulfentrazone applied pre-emergence in soybean. Can. J. Plant Sci. 95: 339-344. Sulfentrazone is a protoporphyrinogen (PPO)-inhibiting herbicide under evaluation for use in soybean in Ontario, Canada. The primary objective of this study was to determine the dose of sulfentrazone applied pre-emergence (PRE) needed to provide 50 and 90% control of redroot pigweed, common ragweed, common lambsquarters and green foxtail. Seven field trials were conducted over a 3-yr period (2007, 2008 and 2009) in southwestern Ontario to evaluate the efficacy of sulfentrazone applied PRE at doses ranging from 26 to 1120 g a.i. ha^-1. The doses of sulfentrazone applied PRE to reduce redroot pigweed, common ragweed, common lambsquarters and green foxtail dry weight by 50% were 104, 139, 15 and 65 g a.i. ha^-1; doses of 241, 514, 133 and 721 g a.i. ha^-1 of sulfentrazone were required for 90% reduction in above-ground biomass of those weed species, respectively. Sulfentrazone applied PRE caused soybean injury only at 560 and 1120 g a.i. ha^-1, with 6 and 13% soybean injury at 4 wk after herbicide application (WAT), respectively. Weed control provided by sulfentrazone applied PRE at a dose of 600 g a.i. ha^-1 was sufficient to maintain 90% of the soybean yield compared with the weed-free control. Therefore, PRE application of sulfentrazone has the potential to provide excellent (>90%) control of selected weeds with minimal to no crop injury; however, weed control varied by species, and thus broad spectrum weed control is not feasible using sulfentrazone alone.