A series of alginate fractions with significant differences in molecular weight and uronic acid compositions were produced by consecutive fractionation and converted to thin and strong cross-linked polymer filaments via extrusion into calcium, aluminum, or polyaluminum (PolyAl) polyvalent solutions followed by drawing and drying. Models were elaborated to relate the alginate uronic acid composition to the tensile performance in both the wet gel filament and the dry filament states. The wet gel model was compared to the theory of the unidirectional elongation of charged polyelectrolyte gels based on the classical rubber elasticity of dilated polymer networks, extended to include the contributions of non-Gaussian chain extensions and the effect of electrostatic interactions. The theory of equilibrium swelling pressure was applied to describe the observed shrinkage of the alginate gels following immersion in a polyvalent solution. Congruent with the theoretical model of charged gels, the tensile performance of the gel filaments prepared from CaCl2 depended on the compositional ratio of guluronic acid dyads in the alginate fraction multiplied by the alginate concentration, while the tensile behavior of wet gel filaments prepared by AlCl3 instead resembled that of elastic solid materials and depended only on the alginate concentration. The dry filament tensile properties were greatly dependent on the preparation conditions, particularly the ratio of stress to alginate concentration and the nature of the ions present during filament drawing. The PolyAl solution effectively caused shrinkage of alginate to a strong extent, and the resulting filaments behaved as highly stiff materials able to withstand stresses of approximately 500 MPa and having elastic moduli as high as 28 GPa.