Aggregation or oligomerization is important for the function of many membrane peptides such as ion channels and antimicrobial peptides. However, direct proof of aggregation and the determination of the number of molecules in the aggregate have been difficult due to the lack of suitable high-resolution methods for membrane peptides. We propose a F-19 spin diffusion magic-angle-spinning NMR technique to determine the oligomeric state of peptides bound to the lipid bilayer. Magnetization transfer between chemically equivalent but orientationally different F-19 spins on different molecules reduces the F-19 magnetization in an exchange experiment. At long mixing times, the equilibrium F-19 magnetization is 1/M, where M is the number of orientationally different molecules in the aggregate. The use of the F-19 spin increases the homonuclear dipolar coupling and thus the distance reach. We demonstrate this technique on crystalline model compounds with known numbers of molecules in the asymmetric unit cell, and show that F-19 spin diffusion is more efficient than that of C-13 by a factor of similar to 500. Application to a beta-hairpin antimicrobial peptide, protegrin-1, shows that the peptide is almost completely dimerized in POPC bilayers at a concentration of 7.4 mol %. Decreasing the peptide concentration reduced the dimer fraction. Using a monomer-dimer equilibrium model, we estimate the Delta G for dimer formation to be -10.2 +/- 2.3 kJ/mol. This is in good agreement with the previously measured free energy reduction for partitioning and aggregating beta-sheet peptides into phospholipid membranes. This F-19 spin diffusion technique opens the possibility of determining the oligomeric structures of membrane peptides.