Neutron small-angle scattering measurements of several different chlorophylls hydrated in deuterated octane-toluene mixtures show that long, hollow cylinders of aggregated chlorophyll are formed. Clear secondary maxima are present in the scattering, and the cylinder diameters are well determined, but depend on the type of chlorophyll. Chlorophyll-a and Bacteriochlorophyll-a were particularly studied, and several samples of each have been measured. Other chlorophylls have also been studied. The results provide strong evidence that chlorophyll cylinders are only certain sizes, with diameters very nearly in the ratio of small integers. Thus, the cylinder diameters appear to be quantized. Neutron scattering results that further test this quantization property are presented here, together with a proposal for the stereochemical features of chlorophyll aggregation which account for the diameter quantization.