Can't say for sure, but I would guess so. The slide is a bit longer on the 45 and the grip is slightly larger, but I don't think either of those things would keep a 40 from fitting in a 45 holster. I would be uneasy about what it might do to the functioning of retention devices on various holsters. I am not sure where all th bells and whistles grab on the the gun in all of the holsters. The smaller ejection port on the 40 could cause problems with some of the systems.