Since the discovery of carbon nanotubes(1), it has been speculated that these materials should behave like nanoscale wires with unusual electronic properties and exceptional strength. Recently, ＇ropes＇ of close-packed single-wall nanotubes have been synthesized in high yield(2). The tubes in these ropes are mainly of the (10,10) type(3), which is predicted to be metallic(4-6), Experiments on individual nanotubes and ropes(7,8) indicate that these systems indeed have transport properties that qualify hem to be viewed as nanoscale quantum wires at low temperature. It has been expected that the close-packing of individual nanotubes into ropes does not change their electronic properties significantly. Here, however, we present first-principles calculations which show that a broken symmetry of the (10,10) tube caused by interactions between tubes in a rope induces a pseudogap of about 0.1 eV at the Fermi level. This pseudogap strongly modifies many of the fundamental, electronic properties : we predict a semimetal-like temperature dependence of the electrical conductivity and a finite gap in the infrared absorption spectrum. The existence of both electron and hole charge carriers will lead to qualitatively different thermopower and Hall-effect behaviours from those expected for a normal metal.