h/Ieasurements on ruthenium of both electrical resistivity (p) and thernlal ' resistivity (PV) extending down to liquid helium temperatures were first reported by White and \Voods (1958). Their two sanlples (Ru2 and Ru3) were both arc-melted but exhibited quite different resistivities. For example, a t 273" I<, the ideal electrical resistivity, p l (= p-PO, po being the residual or impurity resistivity) values were 7.60 and 6.69 pQ cm respectively. This was puzzling, as X-ray studies showed no phases present other than the normal close-packed hexagonal. RIetallographic exanlination did show ". . . some slight evidence for the presence of a decomposed phase . . ." in the less pure sample, Ru2. However, the electrical resistivity determined a t 273O I< on a sintered rod by Justi (1949) gave p l = G.67X10-G pQ cm, in good agreement with the purer sanlple Ru3, and led the authors to conclude that their values for this sanlple were probably representative of ". . . pure close-packed hexagonal ruthenium. . . ." We have recently confi rnled this conclusion by measuring the electrical and thermal resistivities of a high-purity rod kindly lent to us by the International Nickel Company (h4ond) Ltd. This rod, of 4 mrn diameter and 7 lnln long, was prepared from a sintered bar by hot-working and centerless grinding, and was later annealed in vacuo a t 1100" C for 4 hours. The suppliers' analysis of the starting material showed 0.02% 0 s as the major impurity, together with Rh (0.0070/,), Fe (0.0025%), Sb, and I'd (each <0.0010J,). The electrical resistance ratio of 2.5X10-3 confirmed the high purity (cf. Ru3, for which po/p295 % 2.1 X Our thermal resistivity data from 90" K down to 2" K and electrical measurements from 293" I< to 2" K yield values for the ideal resistivities which are very close in magnitude and temperature dependence to those observed for Ru3. The "ideal" values, some of which are given in Table I , are calculated assuming the validity of Matthiessen's rule, viz. p = po+pL and W = Wo+ W,.