Quantization of electrons in solids can typically be observed in microscopic samples if the mean free path of the electrons exceeds the dimensions of the sample. A special case is a quasi one-dimensional metal, in which electrons condense into a collective state. This state, a charge-density wave (CDW), is a periodic modulation of both the lattice and electron density. Here, we demonstrate that samples of K(0.3)MoO(3), a typical CDW conductor, show jumps in conduction, regular in temperature. The jumps correspond to transitions between discrete states of the CDW and reveal the quantization of the wave vector of electrons near the Fermi vector. The effect involves both quantum and classical features of the CDW: the quantum condensate demonstrates modes, resembling those of a classical wave in a resonator. The analysis of the steps allows extremely precise studies of the CDW wave-vector variations and reveals new prospects for structural studies of electronic crystals and fine effects in their electronic states and lattice motions.