The primary structure of a protein consists of a sequence of amino acids and is a key factor in determining how a protein folds and functions. However, conventional methods for sequencing proteins, such as mass spectrometry and Edman degradation, suffer from short reads and lack sensitivity, so alternative approaches are sought. Here, we show that a subnanometre-diameter pore, sputtered through a thin silicon nitride membrane, can be used to detect the primary structure of a denatured protein molecule. When a denatured protein immersed in electrolyte is driven through the pore by an electric field, measurements of a blockade in the current reveal nearly regular fluctuations, the number of which coincides with the number of residues in the protein. Furthermore, the amplitudes of the fluctuations are highly correlated with the volumes that are occluded by quadromers (four residues) in the primary structure. Each fluctuation, therefore, represents a read of a quadromer. Scrutiny of the fluctuations reveals that the subnanometre pore is sensitive enough to read the occluded volume that is related to post-translational modifications of a single residue, measuring volume differences of similar to 0.07 nm(3), but it is not sensitive enough to discriminate between the volumes of all twenty amino acids.