Chemosensory proteins (CSPs) are believed to be involved in chemical communication and perception. A number of such proteins, of molecular mass 13kDa, have been isolated from different sensory organs of a wide range of insect species. Several CSPs have been identified in the antennae and proboscis of the moth Mamestra brassicae. CSPMbraA6, a 112-amino-acid antennal protein, has been expressed in a soluble form in large quantities in the Escherichi coli periplasm. NMR structure determination of CSPMbraA6 has been performed with 1H- and 15N-labelled samples. The calculated structures present an average root mean square deviation about the mean structure of 0.63Å for backbone atoms and 1.27Å for all non-hydrogen atoms except the 12 N-terminal residues. The protein is well folded from residue 12 to residue 110, and consists of a non-bundle α-helical structure with six helices connected by αα loops. It has a globular shape, with overall dimensions of 32Å×28Å×24Å. A channel is visible in the hydrophobic core, with dimensions of 3Å×9Å×21Å. In some of the 20 solution structures calculated, this channel is closed either by Trp-94 at one end or by Tyr-26 at the other end; in some other solutions, this channel is closed at both ends. Binding experiments with 12-bromododecanol indicate that the CSPMbraA6 structure is modified upon ligand binding.