A predicted esterase (EstA) with an unusual new domain from the hyperthermophilic bacterium Thermotoga maritima has been cloned and overexpressed in Escherichia coli. The purified protein was crystallized by the hanging-drop vapour-diffusion technique in the presence of lithium sulfate and polyethylene glycol 8000. Selenomethionine-substituted EstA crystals were obtained under the same conditions and three different-wavelength data sets were collected to 2.6 A resolution. The crystal belongs to space group H32, with unit-cell parameters a = b = 130.2, c = 306.2 A. There are two molecules in the asymmetric unit, with a V(M) of 2.9 A3 Da(-1) and 58% solvent content.