We computed the extension-force dependence for the wormlike model of a polymer chain. The dependence that we obtained differs essentially from the corresponding well-known dependence for the freely jointed chain when the extension exceeds half of the chain contour length. We used the computed results to analyze the measurements of extension of individual DNA molecules under the action of force made recently (Smith, S.B.; Finzi, L.; Bustamante, C. Science 1992, 258, 1122). We took into account the electrostatic interaction between DNA segments for a very low salt concentration. The theoretical dependencies obtained are in very good agreement with the experimental data.