The measured x-ray fluxes of old, nearby pulsars are used to constrain the monopole flux times the cross section for monopole-catalyzed nucleon decay. Observations of PSR 1929 + 10 provide the best limit: $F\times \frac{\sigma v}{(3\mathrm{}\times {10}^{-18\mathrm{}} {\mathrm{cm}}^3 {\mathrm{s}}^{-1\mathrm{}})}<~2\times {10}^{-22\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\mathrm{sr}}^{-1\mathrm{}}$ ${\mathrm{s}}^{-1\mathrm{}}$. When the monopoles captured by the progenitor star while it was on the main sequence are taken into account, the limit becomes $F\times \frac{\sigma v}{(3\mathrm{}\times {10}^{-18\mathrm{}} {\mathrm{cm}}^3 {\mathrm{s}}^{-1\mathrm{}})}<~2\times {10}^{-28\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\mathrm{sr}}^{-1\mathrm{}}$ ${\mathrm{s}}^{-1\mathrm{}}$.

• Received 27 May 1983

DOI:https://doi.org/10.1103/PhysRevLett.51.1625