The structure of the bee venom peptide melittin has been determined in water at pH 6.0, 50 mM sodium phosphate, at various temperatures. At all temperatures studied, the peptide is tetrameric, and consists of two helical regions (residues 2-11 and 13-23) with an unstructured C-terminal region. The connection between the helices (residues 11-13) is well defined. At 30 degrees C, the structure of the monomeric unit has been characterised using NOEs, and a family of structures is presented (root-mean-square deviation to the mean structure 1.4 A over the structured residues), with low NOE violations and good stereochemistry. The angle between the helices is 46+/-13 degrees, and the structure is very similar to the previously determined crystal structure of the aqueous tetramer. The peptide forms a tetramer that is made up from a dimer of dimers. The structure of the dimeric unit has been determined, using a novel joint refinement of intermonomer NOEs and chemical shifts. The relative position of the monomeric units in the dimer is different from that in the crystal, with less direct contact between monomers. As the temperature is raised to 70 degrees C, the peptide remains tetrameric, but the monomer units start to separate, as shown by a reduction in intermonomer NOE intensities and chemical shifts. The structural changes have been characterised: over the temperature range studied, the monomers separate by approximately 2.0 A. This movement may have implications for the mechanism by which melittin inserts into membranes.