Most plants are resistant to herbicides which inhibit acetyl-coenzyme A carboxylase (ACCase) because they have both eukaryotic ACCase and herbicide-insensitive, prokaryotic ACCase. Members of the Gramineae are killed because they have only herbicide-sensitive, eukaryotic ACCase. Here we report that a dicot, Erodium moschatum, is sensitive to the ACCase-inhibiting herbicide haloxyfop because it has herbicide-sensitive ACCase. Erodium moschatum was controlled by haloxyfop application at rates which controlled the gramineous species Digitaria ciliaris and a susceptible Lolium rigidum biotype but did not control the dicot Nicotiana tabacum or a haloxyfop-resistant L. rigidum biotype WLR96. Similarly, the haloxyfop acid concentration required to inhibit activity by 50% in E. moschatum ACCase assays (1.0 μM) was similar to that required for D. ciliaris (2.3 μM) and susceptible L. rigidum (0.4 μM) but much less than that for the resistant L. rigidum biotype WLR96 (353 μM) or the dicots N. tabacum (182 μM) and Pisum sativum (150 μM). Leaf protein extracts from N. tabacum and P. sativum contained both eukaryotic ACCase and prokaryotic subunits of ACCase, but E. moschatum, D. ciliaris and both L. rigidum biotypes exhibited only the eukaryotic ACCase. Thus, the dicot E. moschatum is sensitive to haloxyfop because it lacks the herbicide-insensitive prokaryotic ACCase, a protein that has been considered ubiquitous in dicot species.