The (Ξ, A)-Fleming-Viot process with mutation is a probability-measure-valued process whose moment dual is similar to that of the classical Fleming-Viot process except that the Kingman's coalescent is replaced by the Ξ-coalescent, the coalescent with simultaneous multiple collisions. We first prove the existence of such a process for general mutation generator A. We then investigate its reversibility. We also study both the weak and strong uniqueness of solution to the associated stochastic partial differential equation.[23]) is a coalescent with possible simultaneous multiple collisions. It is then interesting to know whether there exists a generalized Fleming-Viot type probability-measure-valued process whose dual is a function-valued process evolving in the same way as the classical Fleming-Viot dual but with the Kingman's coalescent replaced by the Ξ-coalescent.Such a generalized Fleming-Viot process was first considered by Donnelly and Kurtz [9] and Hiraba [13]. When the spatial motion of the particle is negated, namely, the mutation is 0, it has also been studied by Bertoin and Le Gall ([2], [3], [4], [5]) and Birkner et al [7]. In particular, a special form of such process is constructed in [4] using the weak solution flow of a stochastic equation driven by a Poisson random measure. Generalized Fleming-Viot processes with parent independent jump mutation operators are constructed by Dawson and Li [8] as strong solutions of stochastic equations driven by time-space white noises and Poisson random measures. The classical Fleming-Viot process with Laplacian mutation operator is characterized by Xiong [25] as the strong solution of an SPDE driven by a time-space white noise. The common feature of the approaches of [4], [8] and [25] is to consider the processes of distributions of the measurevalued processes instead of their density processes. In fact, the processes studied in [4] and [8] are usually not absolutely continuous. Similar stochastic equations for Dawson-Watanabe superprocesses have also been studied in [4], [5], [8] and [25].This problem is also studied in the recent work of Birkner et al [6]. When the mutation generator A is the generator for a pure jump Markov process, two constructions of the (Ξ, A)-Fleming-Viot process are found in [6]. One construction is based on modification of the lookdown scheme of [9] applied to exchangeable particle systems for the classical Fleming-Viot process. The (Ξ, A)-Fleming-Viot process arises as the pathwise almost sure limit of the empirical measure for the exchangeable particle system. The other construction is based on the Hille-Yosida theorem. The resulted process gives an example of probability-measure-valued superprocess of jump diffusion type.In this paper, we further study the existence and various properties of this generalized Fleming-Viot process. We first formulate a well-posed martingale problem and show that the (Ξ, A)-Fleming-Viot process X with general mutation generator is the unique solution to such a martingale problem. We then show that X has...