Many amyloid-beta fibril preparations are highly polymorphic, and the conditions under which they are formed determine their morphology. This report describes the application of high-resolution atomic force microscopy (HR-AFM), combined with volume-per-length analysis, to define, identify, and quantify the structural components of polymorphic A beta fibril preparations. Volume-per-length analysis confirms that they are composed of discrete cross-beta filaments, and the analysis of HR-AFM images yields the number of striations in each fibril. Compared to mass-per-length analysis by electron microscopy, HR-AFM analysis yields narrower distributions, facilitating rapid and label-free quantitative morphological characterization of A beta fibril preparations.