A random-matrix formula is derived for the variance of an arbitrary linear statistic on the transmission eigenvalues. The variance is independent of the eigenvalue density and has a universal dependence on the symmetry of the matrix ensemble. The formula generalizes the Dyson-Mehta theorem in the statistical theory of energy levels. It demonstrates that the Universality of the conductance fluctuations is generic for a whole class of transport properties in mesoscopic Systems.PACS numbers: 72.10. Bg, 05.40,+j, 05.60.+w, 74.80.Fp In the sixties, Wigner, Dyson, Mehta, and others developed random-matrix theory (RMT) into a powerful tool to study the statistics of energy levels measured in nuclear reactions [1,2]. It was shown that the fluctuations in the energy level density are governed by level repulsion, which depends on the symmetry of the Hamiltonian ensemble-but is independent of the mean level density [3][4][5][6]. This Universality is at the origin of the remarkable success of RMT in nuclear physics. The universality of the level fluctuations is expressed by the celebrated Dyson-Mehta formula [7] for the variance of a linear statistic A = Σ η α(Ε η ) on the energy levels E n .[The quantity A is called a linear statistic because products of different E n do not appear, but the function a(E) may well depend nonlinearly on E.] The Dyson-Mehta formula readswhere a(k) = J^^dE e lkB a(E) is the Fourier transform of a(E), and β = 1,2, or 4 depending on whether the Hamiltonian ensemble belongs to the orthogonal, unitary, or symplectic symmetry class. Equation (1) shows that (1) the variance is independent of microscopic parameters; and (2) the variance has a universal l/β dependence on the symmetry parameter of the ensemble.In a seminal 1986 paper [8], Imry proposed to apply RMT to the phenomenon of universal conductance fluctuations (UCF), which was discovered diagrammatically by Al'tshuler [9] and Lee and Stone [10]. Shortly afterwards, a RMi of quantum transport was developed by Muttalib, Pichard, and Stone [11]. In this theory the role of the energy levels is played by the transmission eigenvalues T n , or more precisely by the ratio \ n = (l-T n }/T n of reflection and transmission coefficients. Their work is reviewed in Ref. [12], together with a closely related theory due to Mello, Pereyra, and Kumar [13]. (For still another approach, see Ref. [14].) Good agreement was obtained with the diagrammatic theory of UCF. However, it could not be shown that the Universality of the fluctuations is generic for arbitrary linear statistics on the transmission eigenvalues. In particular, no formula with the generality of the Dyson-Mehta theorem could be derived. The lack of such a general theory is being feit especially now that mesoscopic fluctuations in transport properties other than the conductance (both in conductors and superconductors) have become of interest [15][16][17]. The obstacle which prevents a straightforward generalization of the Dyson-Mehta formula was clearly identified by Stone et al. [12]: The cor...