Using a well-established theoretical model, we have addressed the energetic viability of phosphorus nanotubes, and have found that nanotubes based on the black-phosphorus layered structure are indeed stable, having strain energies lower than 0.1 eV/atom for diameters above 1.25 nm. Further, we have found that, if synthesized, these nanotubes would be uniformly semi-conducting and would have a Young's modulus of ca. 300 GPa.