Technical Abstract: Red rice (Oryza sativa L.) is one of the most costly weeds of rice (Oryza sativa L.) in the southern U.S. It is an aggressive competitor and can intercross with commercial rice. Further, mixtures of rice and red rice grain are reduced in value due to the commercially undesirable (red) color of red rice seeds and the expenses incurred by rice mills to remove the unwanted seeds. Physicochemical characteristics of red rice seed and their potential influence on processing and cooking quality are not well known. Thus, more than 150 accessions of red rice (indigenous red rice or red rice X rice crosses) were obtained from the southern U.S. and grown in field plots under flood irrigation at Stuttgart, AR in 2000 and 2001. Grain amylose contents in the U.S. red rice biotypes were higher than commercial medium- and long-grain cultivars and were similar to those of Asian indica germplasm. Amylose levels in red rice crosses, however, were quite variable, ranging from as low as those in commercial medium-grains to as high as those in commercial long-grains. SSR analysis with the RM190 marker confirmed that most U.S. red rice types had an allele (107 or 105 base pairs) that is often present in high amylose cultivars and Asian indicas. Alkali spreading value and protein levels were generally greater for awned than for awnless red rice types, whereas lipid levels were greater for the awnless types. Lipid levels in some red rice types exceeded those in commercial cultivars. The major volatile component of aromatic rice, 2-acetyl-1-pyrroline, was detected in only two red rice accessions, and the genetic source for this trait appeared to be via hybridization. Because of their high levels of amylose, weedy U.S. red rice types are unsuitable for the most common uses of rice in the U.S. and could potentially degrade cooking quality if present in high amounts in any medium-grain or any long-grain that will be processed (i.e. parboiled).