Midguts from Plutella xylostella, which are susceptible (PXS) and highly resistant (PXR) to Bacillus thuringiensis Cry1Ac, were homogenized and reacted with UDP-6-³H-galactose (Gal) or UDP-6-³H-N-acetylgalactosamine (GalNAc) in a bid to find a resistance mechanism. Gal was incorporated into both chloroform/methanol (C/M) soluble and insoluble fractions but GalNAc was incorporated into only the C/M insoluble fraction. PXS demonstrated a significantly higher level of incorporation; in particular, GalNAc was incorporated into the C/M insoluble fraction 1.5 times more in PXS than in PXR. The incorporation of Gal and GalNAc was enhanced with Mn²⁺ 2- and 4-fold, respectively. The tissue homogenate was fractionated after the incorporation of sugars and the cytosol fraction rich in microsome (Micro/Cyto) had the highest radioactivity. Although the incorporation of these two sugars was higher in PXS, the level of incorporation was at best 2-fold higher. Mutation in fundamental steps of oligosaccharide synthesis must be lethal, thus, the mutation must be strictly limited to that bringing the different sugar residue(s) to the outer region of the oligosaccharide. This must be the reason why the lower sugar transferase activity in PXR could not explain the substantially higher resistance in PXR. We briefly discuss the mechanism of resistance in P. xylostella against Cry1Ac.