A new betaine compound, ( N-carboxymethyl)dimethylammoniacetate, Me 2N(CH 2CO 2H)CH 2CO 2 1 (space group C2 c with a = 5.748(2), b = 11.454(3) c = 11.515(4) A ̊ , β = 90.29(2)°, Z = 4 and R = 0.041) and its hydrochloride Me 2N(CH 2CO 2H)CH 2CO 2·HCl 2 (space group P2 1 c with a = 6.810(2), b = 10.392(3), c = 13.058(4) A ̊ , β = 95.39(3)°, Z = 4 and R = 0.033) have been synthesized and characterized by X-ray crystallography. In compound 1 adjacent [Me 2N(CH 2CO 2) 2] − moieties residing on sites of symmetry 2 are bridged by an acidic proton to form a very strong symmetrical O … H … O hydrogen bond of length 2.453(3) Å, thus giving rise to a polymeric zigzag chain in the direction of the c axis. Likewise in compound 2 the carboxyl groups of two adjacent [Me 2N(CH 2CO 2H) 2] + molecules are bridged by a chloride anion to form a (OH … Cl … HO) hydrogen bond, yielding a zigzag chain running parallel to the b axis.