The volume-dependence of the structure factors of the one component plasma (OCP) and of the hard sphere fluid (HSF) are calculated from data derived from computer experiments. In the OCP case the results are in essential agreement with the predictions of a simple formula due to Egelstaffet al.which the latter writers have shown to apply to rubidium. This provides further evidence that the OCP is a suitable reference system for the liquid alkalis. The results for the HSF, as expected, do not obey the formula of Egelstaffet al.and presumably are more representative of polyvalent liquid metal behaviour.