BenM, a member of the LysR-type family of transcriptional regulators, controls genes for benzoate degradation in the Gram-negative bacterium Acinetobacter sp. strain ADP1. Recent studies show that BenM activates benABCDE expression synergistically in response to two effector ligands: cis,cis-muconate (CCM) and benzoate. As an initial step in investigating the structural basis of dual effector response, the effector-binding domain of BenM (BenM-EBD) was crystallized by the microbatch-under-oil technique with conditions optimized from high-throughput screens performed by the Hauptman-Woodward Institute. Data-collection quality crystals of BenM-EBD belonged to space group P2(1)2(1)2(1), diffracted to 2.3 A and had unit-cell parameters a = 65.64, b = 66.34, c = 117.46 A. The influence of effector ligands on crystal formation was also evaluated. The presence of benzoate or CCM impaired the formation of crystals. The presence of both effectors together resulted in a dramatic decrease in the production of crystals. The effector-binding domain of CatM, a homolog of BenM, was also crystallized.