The glycosylation alterations of mouse small intestinal mucins during a 12-day infectious cycle caused by the parasite Nippostrongylus brasiliensis have been studied. The guanidinium chloride insoluble mucins were isolated at day 0 to 12 from the small intestine of infected and non-infected C57BL/6 mice. The O-linked oligosaccharides were released by reductive beta-elimination from the mucins and separated into neutral, sialylated and sulfated fractions. All fractions were analyzed by monosaccharide composition analysis and the neutral oligosaccharides were structurally characterized by gas chromatography/mass spectrometry. Two oligosaccharides containing blood group H-type epitopes (Fucalpha1-2Gal-) were transiently expressed with a maximum at day 6. Additional oligosaccharides with the common structure HexNAc-Gal-3GalNAcol were transiently induced with a maximum at day 10. Northern blot analysis on total RNA showed a transient expression at day 4-6 of the Fut2 gene encoding a Fucalpha1-2 fucosyltransferase, probably responsible for the detected blood group H-type epitopes. Comparisons with the corresponding infection in rat studied previously, revealed structurally different alterations, although occurring as transient events in both species. Both showed an induced blood group-type transferase halfway through the infection (a blood group A transferase in rat) and an induced transferase adding a terminal GalNAc (to a sialic acid- containing epitope in rat) towards the end of the infection. These differences between closely related species suggest rapid evolutionary alterations in glycosyltransferase expression.