Dimeric (R)-specific enoyl-coenzyme A (CoA) hydratase from Aeromonas caviae catalyzes the hydration of trans-2-enoyl-CoAs with carbon lengths of 4-6 to yield their corresponding (R)-3-hydroxyacyl-CoAs and is essential for polyhydroxyalkanoate (PHA) biosynthesis. The enzyme has been crystallized by vapour diffusion against a reservoir solution containing 20% polyethylene glycol 4000, 5% 2-propanol and 20 mM HEPES pH 7.0 at 298 K. Crystals belong to the monoclinic space group C2, with unit-cell parameters a = 111.54 (3), b = 59.29 (1), c = 47.27 (4) A, beta = 113.04 (2) degrees and contain a dimeric molecule in the asymmetric unit. Flash-cooling of a crystal at 100 K alters its unit-cell parameters to a = 109.82 (7), b = 57.98 (6), c = 46.84 (2) A, beta = 112.71 (3) degrees. Native data to a resolution of 1.7 A have been collected with 94.5% completeness and an R(merge) of 4.0% under cryogenic (100 K) conditions using synchrotron radiation.