The weedy populations of "green" foxtail grasses (Setaria spp.) in our area have become increasingly difficult to interpret since the introduction of Setaria faberi Herrm. Conventional keys do not well separate this species from S. iridis (L.) Beauv., and some authors have questioned their distinctness. However, S. viridis is diploid (n 9) and S. faberi is tetraploid (n 18). Li, Pao, and Li (1942) attempted hybridization between diploid and tetraploid species of Setaria and found almost complete sterility in their crosses. The two hybrids which they succeeded in making were S. faberi X S. italica and (S. itatica X S. viridis) X S. faberi. Like S. viridis, S. itatica is diploid (n 9). Both crosses were nearly completely sterile, as might be expected in crosses between diploids and tetraploids. Long a common weed species in the United States, S. viridis has---herbage which is glabrous except for slight ciliation on the upper margins of the sheaths. The prevalent form of S. faberi bears soft pubescence on the upper leaf surfaces only and is usually distinguished from other species on this basis. Recently we have discovered in Iowa two other variants of S. faberi, differing from the common form in pubescence pattern, but having spikelets identical with those of the widespread type, and having the same chromosome number (Fig. 1, 1-3).