of the blades 14 the root 20 has a greater circumferential width than the outer edge 18 i e the cross section of each blade 14 tapers towards the well bore contacting periphery of the centraliser 10 the individual and collective shapes of the blades 14 and of the longitudinal fluid flow passages defined between adjacent pairs of the blades 14 gives the centraliser 10 improved flow characteristics and minimises the build up of trapped solids during use of the centraliser 10 longitudinally opposite ends of the blades 14 and of the body 12 are chamfered to assist in movement of the centraliser 10 up down a well bore although the blades 14 are shown separately from the body 12 in fig4 and 5 and while the blades 4 could be separately formed and subsequently attached to the body 12 by any suitable means it is preferred that the centraliser body 12 is fabricated as a one piece article preferably by being precision cast in a suitable metal or alloy the blades 14 in the first embodiment have strips 19 of polytetrafluoroethylene ptfe attached to their outer surfaces 18 to bear against the inner surface of the well bore the ptfe strips are glued or otherwise attached to the blades no modification is necessary for the blades to receive the strips 19 but strip attachment plates not shown can be provided on the outer surfaces 18 if desired to improve the ability of the strip 19 to attach to the particular metal etc.