X-ray fiber diffraction patterns of the sodium salt of l-carrageenan correspond to three (1, 11, and 111) distinct packing arrangements of half-staggered, 3-fold, double helices having nearly the same helical repeat (c-axis 13.0-13.2 angstrom). While two of the three forms crystallize in trigonal nets with a = 24.02 for I and 21.8 angstrom for III, II favors an orthogonal net with a = 13.70 and b = 20.08 angstrom. Detailed structure analysis of I has shown that three helices in the unit cell, each pair at 13.9 angstrom apart, interact only through their 2-sulfate and 4-sulfate groups with the aid of sodium ions and ordered water molecules [Janaswamy, S.; Chandrasekaran, R. Carbohydr. Res. 2001, 335, 181-194]. The ability of sodium to induce two other crystalline structures, in which the helix-helix separations are considerably shorter (12.2 in II and 12.6 angstrom in III), suggests that interhelical interactions vary significantly among the polymorphs. These observations have ramifications to the practical applications of l-carrageenan in food and pharmaceutical industries.