A theory of the magnetoresistance of disordered carbons is presented and applied to experimental results on carbon fibers and glassy carbon. The important features of the model include (a) a two-dimensional band structure and density of states, (b) a shallow acceptor level due to defects, (c) collision-broadened Landau levels, and (d) an extra density of states in the m=0 Landau level to account for the effects of partial three-dimensional ordering of the graphitelike layers. The negative magnetoresistance results from field-induced changes in the density of states which lead to an increase in the carrier concentration with field. Four structure-dependent parameters are used to fit the resistivity as a function of temperature, field strength, and orientation of the sample in the field. Approaches to improving the model are discussed. © 1979 The American Physical Society.