A marking device for a generally cylindrical conductor of fluids, electricity, light waves and the like includes a hollow shell containing a horizontal longitudinal through passage for receiving one of a plurality of conductors of different diametric sizes, the shell being horizontally divided to define an upper shell section having a lower internal surface provided with a conductor seat, and a lower shell section including an internal resilient support device for biasing the conductor laterally upwardly toward the seat. An indicia-bearing plate is connected with the external surface of the upper shell section directly above the conductor seat. The conductor seat, the indicia-bearing plate, and the biasing device are symmetrically arranged relative to the vertical plane passing longitudinally through the shell, whereby the biasing forces are uniformly balanced for conductors having different diametric sizes.