Results of resistivity measurements on a series of pure gallium single crystals with size-limited residual scattering exhibit deviations from Matthiessen's rule (DMR) that are compared with those observed in single-crystal gallium alloys with impurity-limited residual scattering. Such a comparison provides a test of the isotropisation idea which has been used to explain the deviations in a number of metals. The high degree of correlation between the results in the two cases, when considered as a function of residual resistivity, tends to indicate that some other mechanisms is responsible for the DMR in this metal.