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Conventional electrorheological (ER) fluids consist of suspensions of microparticles in usually nonconducting fluids with a different dielectric constant. The interparticle interaction, and hence, the rheology of ER fluids, is determined by an external electric field, which polarizes grains and thus induces additional dipole-dipole coupling. The electric field plays a role of a new degree of freedom that allows us to ¿¿tune¿¿ the interaction between particles. This makes the phase diagram of ER fluids remarkably diversified. Here, we report on the experimental investigations of ¿¿ER plasmas,¿¿ where the control of the interparticle interaction by an externally applied electric field is due to distortion of the Debye spheres that surround microparticles in a plasma. Interactions in ER plasmas under weak ac fields are mathematically equivalent to those in conventional ER fluids. Microgravity experiments as well as molecular dynamics simulations show a phase transition from an isotropic to an anisotropic plasma states as the electric field is increased. The variational Gibbs-Bogoliubov approach allows us to recover the phase diagram of ER plasmas, which includes string fluids and anisotropic solid phases.