Technical Abstract: Experimental fields near Stuttgart, AR that had been seeded with red rice and the imidazolinone resistant rice (IMI rice) lines, CF2551 (CL121), CF3291 (CL141), or CF0051 in 2000, were sprayed with imazethapyr in 2001. These applications spared herbicide resistant seedlings that potentially resulted from outcrossing between red rice and IMI rice. Strawhull awnless red rice was the predominant type in these fields. DNA was extracted from numerous surviving seedlings. In CF2551 plots in which red rice and rice had flowered synchronously, 49 plants were considered probable F1 hybrids based on the presence of rough leaves and red colored seeds, and DNA fingerprints that were consistent with both IMI rice and strawhull red rice. Thirty-six probable F1 hybrids were detected from CF0051 plots in which flowering of rice and red rice overlapped slightly. Only 6 survivors from CF3291 plots in which rice flowered several weeks after red rice, were probable F1 hybrids. In 2000, red rice produced an estimated 3.7 million seeds per plot. Thus, the outcrossing rates between CF2551, CF3291, and CF0051 IMI rice and red rice are estimated at 0.0013%, 0.00016%, and 0.00097%, respectively.