Enzyme transition-state mimics can act as powerful inhibitors and allow structural studies that report on the conformation of the transition-state. Here, mannoimidazole, a mimic of the transition state of mannosidase catalyzed hydrolysis of mannosides, is shown to bind in a B_2,5 conformation on the Clostridium perfringens GH125 α-1,6-mannosidase, providing additional evidence of a ^OS₂–B_2,5–¹S₅ conformational itinerary for enzymes of this family.