The expression of a protein-coding DNA fragment in a microorganism such as Escherichia coli requires that the exogenous DNA segment be inserted precisely in phase with bacterial translation initiation signals. We report the construction of derivatives of the M13 vectors M13mp7 and M13mp701 in which a HindIII site has been inserted, within the N-terminal section of the beta-galactosidase gene, in all three phases of translation. These vectors may thus be used for the expression, under the control of the lac promoter and translation initiation signals, of protein coding DNA segments flanked by a HindIII site. In all cases the insertion of a DNA fragment can be recognized by the abolition of beta-galactosidase activity. These vectors have been used to direct the expression, in E. coli, of a cDNA segment coding for the rabies virus surface glycoprotein. The proteins produced have been shown to react with antisera raised against authentic rabies glycoprotein.