The invention relates to polymer particles in the form of filaments formed by block copolymers, as well as to the use thereof as rheology modifiers for aqueous or organic solutions. A first aspect of the invention relates to filamentous polymer particles having a length/diameter ratio greater than 100, said particles being formed by block copolymers synthesized by controlled radical emulsion polymerization in emulsion, performed using at least one hydrophobic monomer in the presence of a water-soluble marcoinitiator. The invention is characterized in that the filamentous particles are obtained in an aqueous medium for the synthesis of said block copolymers, performed by heating the reaction medium to a temperature of 60 to 120° C., the final block copolymer containing between 10 and 50 mol. % of the water-soluble macroinitiator and the hydrophobic monomer conversion rate being at least 50%. A second aspect of the invention relates to a method for modifying the rheology of an organic or aqueous solution using said filamentous polymer particle.