Complementation can be defined as the process by which the function affected by a mutation is provided in trans by fully competent genotypes. Complementation can, thus, counter the effects of selection of deleterious mutants. Complementation of mutants defective for replication, movement and transmission has been often described in experiments with viruses and, occasionally, has been reported to occur in their natural populations. However, the role of complementation in virus evolution has been overlooked. Here is provided a quantitative estimate of the efficiency of complementation, defined as the probability that a non-functional mutant accomplishes a function relative to a functional one. For this, the frequency of mutants of Tobacco mosaic virus (TMV) defective for cell to cell movement was estimated in wild type tobacco plants and in transgenic plants expressing the TMV movement protein (MP) from a transgene. Mutants lethal for cell-to-cell movement were complemented by wild-type TMV in the first case, and by the transgene-expressed MP in the second case. Assuming that complementation is fully efficient in the transgenic plants, a value for the efficiency of complementation of 0.34 was obtained. Thus, complementation can efficiently counter selection on lethal mutants, and may have an important role on virus evolution. Complementation may be relevant for management of viral diseases if the complemented deleterious mutation is linked to other functions affecting the pathogenicity or epidemiology of the virus.