The chemical composition of a number of silk fibres of different origins, and of the Byssus fibre of Pinna nobilis, has been determined by chromatographic analysis. The proportion in the silks of amino acids with long side-chains to those with short side-chains, enables them to be placed into three main groups. These groups are typified by the silk obtained from the Anaphe moth (Anaphc sp.), from the Silk moth (Bombyx sp.), and from the Tussah moth (Antheraea sp.). The value of the ratio of long side-chain amino acids to short side-chain amino acids, multiplied by 100, varies from 3.2 to 9.3 for the Anaphe group, (mean 5.5) ; from 22.1 to 30.0 for the Tussah group (mean 26.6) ; and is 13.5 for Bombyx mori. For Byssus fibres the value is 156. Some physical properties of these fibres have been investigated, and correlated where possible with the amino-acid analyses. Load-extension characteristics and elastic properties or the fibres in air at 65 per cent r.h. and 20°C, and in water at 20°C and 95°C, can be correlated with the distribution of amino acids. The moisture regain of the fibres at 25°C appears to depend mainly on the peptide link of the fibroins, and is consequently not much affected by amino-acid distribution. The apparent density of the fibres changes when measured in benzene and then in water, and this change is related to the ease with which water can penetrate the fibres and, therefore, to the proportion of amino acids with bulky side-chains.