An efficient two-dimensional magic-angle-spinning (MAS) NMR technique to selectively detect alpha -sheet residues in proteins is demonstrated. The method exploits the relative orientation between N-H and C alpha -H alpha dipolar couplings, that is, the phi torsion angle, in each amino acid residue. The different phi angles between the alpha -helical and beta -sheet conformations give rise to distinctly different dipolar modulations. After one-half of a rotation period of dipolar evolution, beta -sheet residues retain significant intensities (20-50%), while alpha -helical signals vanish for suitable spinning speeds. Thus, a distinction between sheet and helix geometries can be achieved without lengthy measurements of the entire dipolar modulation curve. Combining this constant-time dipolar modulation with 2D N-15-C-13 correlation, we selected the beta -sheet signals and removed the alpha -helical resonances in mixtures of model amino acids N-acetyl valine and N-tBoc alanine. To demonstrate the suitability of this technique for solid proteins, beta -sheet filtration was applied to C-13- and N-15-labeled ubiquitin. Two pulse sequences that are suitable for slow (similar to4 kHz) and faster spinning speeds (similar to7 kHz) are compared. The technique represents a complementary approach to the recently introduced a-helix filter experiment (Hong, J. Am. Chem. Sec. 122 (2000) 3762-3770) and is expected to further enhance the efficiency and confidence of protein structure determination by solid-state NMR.