The electrical transport properties of single-wall carbon nanotubes are shown to be strongly influenced by the presence of transition-metal impurities derived from the catalyst introduced to stimulate their growth. Data on thermoelectric power and electrical resistance in the temperature range 10‐400 K were obtained on a series of samples prepared using M-Y catalysts ~M5Cr, Mn, Co, Fe, Ni!. The unusual transport behavior observed is tentatively assigned to an interaction between the magnetic moment of the M atom and the spin of the conduction p electrons of the nanotubes, i.e., the Kondo effect. @S0163-1829~99!50240-3# Small diameter (d,2 nm) single-wall carbon nanotubes ~SWNT! have now been prepared in sufficient yield from catalyzed carbon plasmas to allow the widespread study of the chemical and physical properties of these molecular filaments. 1 In this work, we present the results of a systematic study of the transport properties of SWNT mats produced from a series of transition-metal‐Y catalysts which show that the identity of the catalyst has a pronounced effect on the electrical transport. It is known from numerous studies on nonmagnetic metallic hosts ~X! that only ;10‐100 ppm magnetic impurity ~M! can have a dramatic effect on the electrical transport properties of the host. 2 A giant peak in the thermopower vs temperature and an associated resistivity minimum were reported in the systems X: M, where X5Au, Ag, Cu, and M 5Ti, V, Cr, Mn, Fe, Co, Ni. 2,3 These observations are explained theoretically as the result of an interaction between the magnetic moments of the impurity atoms and the conduction electron spins of the host. 2,3 This interaction was shown to lead to a new spin-dependent scattering mechanism and a narrow hybridization peak or ‘‘Kondo resonance’’ in the electronic density of states positioned near the Fermi energy E F . 3,4 The Kondo temperature can be written approxi