Abstract The electron-impact (e.i.) and chemical-ionization (c.i.) mass spectra of the 2-di- N-methyl ( 2), 2- N-acetyl ( 3), and 2-( N-acetyl)- N-methyl ( 4) derivatives of 1,5-di- O-acetyl-7- O-(2-amino-2-deoxy-3,4,6-tri- O-methyl-α- d-glucopyranosyl)-2,3,4,6-tetra- O-methyl- l- glycero- d- ma nno-heptitol, obtained from the methylation analysis of the core oligosaccharides of Aeromonas hydrophila Chemotypes I and II, are described and their fragmentation patterns proposed. The e.i.-mass spectrum of the 2-dimethylamino derivative 2 showed the primary fragment ion A 1, characteristic of the glycosyl group of this glycosylalditol, whereas the mass spectra of the 2-acetamido ( 3) and 2-( N-methylacetamido) derivatives ( 4) gave the respective primary ions, A 1-fragments, and ald +-fragments diagnostic of the glycosyl group and the alditol residue of the respective glycosylalditol. The c.i.-mass spectra of the glycosylalditol derivatives 2, 3, and 4 gave, as major peaks, the protonated molecular ion [MH] +, together with the primary fragment ion A 1 characteristic of the glycosyl group of the respective derivatives. The e.i.- and c.i.-mass spectra and fragmentation pattern of methyl 7- O-(2-acetamido-3,4,6-tri- O-acetyl-2-deoxy-α- d-glucopyranosyl)-2,3,4,6-tetra- O-acetyl- l- glycero-α- d- manno-heptopy ranoside ( 6) are also reported.