An increase with time in resistivity occurs in plutonium at low temperatures as a result of radioactive self damage. The effect has been studied in experiments at liquid helium temperature lasting over 8000 h and has been found to exist in the $\alpha$, $\beta$ and $\delta$ phases of the metal. In all cases the resistivity approaches saturation, the rates of increase depending on the crystal structure and isotropic content of the sample. In spite of wide differences in these rates the functional increase is roughly the same. The temperature dependence of the accumulated resistivities in all three modifications deviates very strongly from a simple additive resistance obeying Matthiesen's rule. The resistivity of fully damaged $\alpha$ plutonium is almost temperature independent, becoming smaller than that of the undamaged specimen above about 50 $^\circ$K. Measurements on neptunium and uranium 233 have also shown resistivity increases with time at helium temperatures, but the effect is too small to decide whether Matthiesen's rule is obeyed in these metals. The observed effects have been discussed under the assumption that in plutonium scattering of electrons at low temperatures is mainly due to a co-operative phenomenon.