FadR, an acylCoA-dependent Escherichia coli transcription factor controlling the expression of genes involved in fatty-acid degradation and synthesis, has been crystallized. Crystals of two binary complexes were obtained. The FadR-CoA complex crystallized in space group C222(1), with unit-cell parameters a = 61.3, b = 102.0, c = 91.3 A. The FadR-octanoyl-CoA complex crystallized in space group P6(5)22, with unit-cell parameters a = b = 59.7, c = 296.2 A. Both crystal forms diffracted to 3.5 A on a rotating-anode generator. In both crystal forms, the asymmetric unit contains one subunit. The protein is known to be a homodimer; each subunit consists of two domains of unknown fold. For the acyl-CoA-binding domain, a previously undetected sequence homology to PAS domains, in particular the photoactive yellow protein, is reported.