The solution conformation of the Ascaris trypsin inhibitor, a member of a novel class of proteinase inhibitors, has been investigated by nuclear magnetic resonance spectroscopy. Complete sequence-specific assignments of the 1H NMR spectrum have been obtained by using a number of two-dimensional techniques for identifying through-bond and through-space (less than 5-A) connectivities. Elements of regular secondary structure have been identified on the basis of a qualitative interpretation of the nuclear Overhauser enhancement, coupling constant, and amide exchange data. These are two beta-sheet regions. One double-stranded antiparallel beta-sheet comprises residues 11-14 (strand 1) and 37-39 (strand 2). The other triple-stranded sheet is formed by two antiparallel strands comprising residues 45-49 (strand 4) and 53-57 (strand 5) connected by a turn (residues 50-52), and a small strand consisting of residues 20-22 (strand 3) that is parallel to strand 4.