The D-gluco-isoquinuclidines 3 and 4 were prepared and tested as inhibitors of the β-glucosidases from Caldocellum saccharolyticum and from sweet almonds; the results are compared to the inhibition of snail β-mannosidase by the D-manno-isoquinuclidines 1 and 2. Exploratory experiments in the racemic series showed that treatment of the ester epoxide 6 with benzyl alcoholates leads only to epimerisation, transesterification, and formation of the cyclopropane 9. Ring opening of the reduced epoxide 13 by NaN3 proceeded regioselectively to provide 14. Treatment of the C(6)[BOND]O-triflate 16 with AcOCs induced a rearrangement; the reaction with NaN3 gave the C(5)-azido derivative 14. The acetoxy triflate 18, however, reacted with AcOCs to provide the desired gluco-isoquinuclidine 19. Similarly, the enantiomerically pure acetoxy triflate 22 provided the D-gluco-isoquinuclidine 24, which was reduced and deprotected to provide 3 and 4. The deoxy analogues 30 and 31 were obtained by reductive deiodination of the iodide 27, derived from 22. The D-gluco-isoquinuclidines 3, 4, 30, and 31 are much weaker inhibitors of β-glucosidases than the D-manno-analogues 1 and 2 of snail β-mannosidase. The N-benzyl derivative 3 is a weaker inhibitor than the N-unsubstituted analogue in the gluco-series, while it is a much stronger inhibitor in the manno-series. A consideration of the pKHA values of the isoquinuclidines 14 and the pH value of the enzyme assays suggests that the D-gluco-isoquinuclidines are poor mimics of the shape of a reactive, enzyme-bound gluco-conformer, while the D-manno-analogues are reasonably good mimics of a reactive, enzyme-bound manno-conformer. The inhibition results may also suggest that the glycosidase induced lengthening of the scissile bond and rehybridisation of the anomeric centre are more strongly correlated with the change of the ground-state conformation during hydrolysis of β-D-glucopyranosides than of β-D-mannopyranosides.