Fibrous hydrotalcites having a hexagonal needle-like crystal structure. The fibrous hydrotalcites are produced by contacting a basic magnesium compound having a needle-like crystal structure and expressed by the following formula (2)Mg(OH).sub.2-n'x.sbsb.2 A.sub.x.sbsb.2.sup.n'.spsp.-.m.sub.2 H.sub.2 O (2)wherein A.sup.n'.spsp.- represents a monovalent or divalent anion, n' is 1 or 2, and x.sub.2 and m.sub.2 are numbers satisfying the following conditions,0.2.ltoreq.x.sub.2 .ltoreq.0.5,O.ltoreq.m.sub.2 .ltoreq.2,with a compound capable of providing a trivalent metal cation and being soluble in a liquid reaction medium which is chemically inert and is a non-solvent for the basic magnesium compound, the contacting being carried out in said liquid reaction medium under conditions which do not cause a loss of the needle-like crystal form of the basic magnesium compound, while maintaining the ratio of M.sup.3+ to the sum of Mg and M.sup.3+ at O