The herbicide rimsulfuron has demonstrated poor crop selectivity, particularly with certain corn hybrids. Because hybrid screening under field conditions is expensive and time consuming, an alternate method was investigated in the laboratory. Field experiments were conducted to determine the tolerance of 38 corn hybrids to rimsulfuron. Postemergence applications of 12.5, 25 and 50 g ai/ha, were applied to hybrids at the 6-7\spth leaf stage. Observations were taken 7, 14 and 21 DAT. Tolerant hybrids such as Pioneer Brand 3751 LR corn exhibited as little as 0% injury, while sensitive hybrids such as Pioneer Brand 3897 displayed injury symptoms as high as 75% at 50 g ai/ha. The same 38 corn hybrids were evaluated in the laboratory, using a growth-room whorl bioassay. At the 6\spth leaf stage, corn plants were treated with 0.625, 1.25 and 2.5 μg ai of formulated rimsulfuron. Applications were made into the whorl of the plants with a repeater pipette. All hybrids identified as rimsulfuron-susceptible using the field screening method were identified in the laboratory as susceptible with this bioassay. A negative correlation between crop heat unit requirement and sensitivity to rimsulfuron was confirmed. The growth-room whorl bioassay proved to be a fast, easy and reliable method to identify rimsulfuron-susceptible corn hybrids.