Dresserite occurs with weloganite in cavities in an alkalic sill intruding limestone at St-Michel, Montreal Island, Quebec. Chemical analysis corresponds to Ba (sub 1.98) Al (sub 4.06) C (sub 4.06) O 23 H (sub 13.67) , ideally Ba 2 Al 4 (CO 3 ) 4 (OH) 8 X3H 2 O. This is the barium analog of the lead mineral, dundasite. Dresserite occurs as white spherical aggregates averaging about 2 mm in diameter, each of tapering fibers elongated parallel to c. It is biaxial negative with 2V = 30 degrees to 40 degrees ; extinction is parallel. The mineral is orthohombic a = 9.27, b = 16.83; c = 5.63 A; the space group can be Pbmm, Pb2 1 m, or Pbm2. The X-ray powder pattern superficially resembles that of dundasite. Strongest pattern lines and a complete listing of observed spectra are given with those of dundasite for comparison.