Auxin is an important plant hormone that plays significant roles in plant growth and development, influencing apical dominance, hypocotyl elongation, lateral root initiation, gravitropism, and phototropism. There are two forms of endogenous auxin, indole-3-butyric acid (IBA) and the more abundant indole-3-acetic acid (IAA). Auxin signal transduction pathways remain to be fully elucidated, though numerous auxin-response mutants have been identified. IBR5 is a gene potentially involved in auxin signal transduction. The ibr5 mutant has pleiotropic defects including decreased sensitivity to the inhibitory effects of auxin, synthetic auxins, auxin transport inhibitors, and the phytohormone abscisic acid. Like certain other auxin-response mutants, ibr5 has a long root and short hypocotyl when grown in the light. Additionally, ibr5 displays aberrant vascular patterning, increased leaf serration, and reduced accumulation of an auxin-inducible reporter. Positional information was used to determine that the gene defective in ibr5 encodes an apparent dual-specificity phosphatase. Using immunoblot and promoter-reporter gene analyses, we found that IBR5 is expressed throughout the plant. The identification of IBR5 relatives in other flowering plants suggests that IBR5 function is conserved throughout angiosperms. The results suggest that IBR5 is a phosphatase modulating phytohormone signal transduction and support a link between auxin and abscisic acid signaling pathways. Less is known concerning IBA function, though this auxin is widely used for rooting in commercial and agricultural settings. IBA functions primarily via its conversion to IAA by a process similar to fatty acid beta-oxidation in the peroxisomes. Additionally, IBA may act via its own signaling pathway, separate from IAA. Screens for mutants resistant to the inhibitory effects of IBA that remain sensitive to IAA have revealed 32 mutants to date. These IBA-response mutants have been placed into five distinct classes based on phenotypic analyses of root and hypocotyl elongation following growth on various hormones and unsupplemented medium. Here I characterize eight IBA-response mutants and use positional information to localize the genes defective in these mutants. Analyzing genes involved in IAA and IBA responses will provide a better understanding of the function of auxin in Arabidopsis thaliana and may eventually allow manipulation of the pathways involved to enhance agricultural production.