The three-dimensional structure of a 42-residue fragment containing the N-terminal EGF-like module of blood coagulation factor X was determined by means of 2D NMR spectroscopy and computer simulation. The spectroscopic data consisted of 370 NOE distances and 27 dihedral angle constraints. These were used to generate peptide conformations by molecular dynamics simulation. The simulations used a novel functional form for the constraint potentials and were performed with two time steps to ensure rapid execution. Apart from preliminary runs to aid assignment of NOEs, 60 runs resulted in 13 accepted structures, which have two antiparallel § sheets, no a helices, and five tight turns. There is no hydrophobic cluster. The root mean square deviation for the backbone of the 13 conformations is 0.65 ± 0.11 A against their mean conformation. About half of the side chains have well-defined structure. The overall conformation is similar to that of murine EGF.