The author characterizes generic groups, namely Zariski dense subgroups (not necessarily discrete) of the isometry group of a symmetric space of noncompact type in terms of the marked length spectrum. The main result reads as follows: Let \(X\) and \(Y\) be symmetric spaces of noncompact type without Euclidean de Rham factor. Let \(\Gamma_1\) and \(\Gamma_2\) be Zariski dense subgroups of the real semisimple Lie groups \(\text{Iso}^0(X)\) and \(\text{Iso}^0(Y)\), respectively. If there is a surjective homomorphism \(\phi:\Gamma_1\to\Gamma_2\) preserving the translation lengths of isometries, i.e. such that \(l(\gamma)=l(\phi(\gamma))\) for all \(\gamma\in\Gamma_1\) where \(l(\gamma)=\inf_{x\in X}\{ d(x,\gamma x)\}\), then \(X\) is isometric to \(Y\) and \(\Gamma_1\), \(\Gamma_2\) are conjugated by an isometry. As an application, the author obtains the following affirmative answer to a Margulis's question: Let \(G\) be a higher rank real semisimple Lie group of noncompact type, let \(\Gamma_1\), \(\Gamma_2\) be Zariski dense subgroups, let \(\phi: \Gamma_1\to\Gamma_2\) be a surjective homeomorphism, and let \(\log\lambda(\gamma)=k(\gamma)\log\lambda(\phi(\gamma))\) for all \(\gamma\in\Gamma_1\), where \(\lambda(\gamma)\) is a unique element in a Weyl chamber \(A^+\) which is conjugated to the hyperbolic component of \(\gamma\) in the Jordan decomposition and \(\log\) is a natural map from a Lie group \(A^+\) to its Lie algebra. Then \(\Gamma_1\) and \(\Gamma_2\) are conjugate. More elementary and algebraic proofs of the above cited theorems are given in [\textit{F. Dal'Bo,} and \textit{I. Kim}. Comment. Math. Helv. 77, No. 2, 399--407 (2002; Zbl 1002.22005)]. In the paper under review, the author uses more general geometric methods which can be possibly used for general Hadamard manifolds and deduces the following two corollaries: (1) Let \(\phi:\Gamma_1\to\Gamma_2\) be an isomorphism between Zariski dense subgroups in real semisimple Lie groups. Then the following are equivalent: (a) \(\Gamma_1\) and \(\Gamma_2\) are conjugate; (b) \(\Gamma_1\) and \(\Gamma_2\) have the same marked length spectrum, i.e. there is an isomorphism preserving the translation lengths of an isometry; (c) \(\nu(g,h)=\nu(\phi(g),\phi(h))\) for any hyperbolic elements \(g,h\in\Gamma_1\) where \(\nu\) stands for a cross-ratio. (2) A map \(l:R_{npnk}\to\mathbb{R}^\Gamma\) defined by \(\rho\to(l(\rho(\gamma)))_{\gamma\in\Gamma}\) is an injection. Here \(R_{npnk}\) is the space of nonparabolic representations modulo conjugacy, from a group \(\Gamma\) into a real semisimple Lie group of noncompact type without a center, whose Zariski closures do not contain any compact factor.