NEW LIMITS ON THE MASS OF MAGNETIC MONOPOLES have been established in an experiment at Fermilab. The equations written down by James Clerk Maxwell in the 19th century are not symmetric with respect to electric and magnetic forces. They can be made symmetric if there exists a magnetic monopole, a particle, comparable to the electron, with an isolated north or south magnetic pole (all known magnets are dipoles, possessing both south and north poles). It is customary when searching for particles that are rare or heavy to recreate artificially the conditions that prevailed in the early universe, and that means smashing beams of particles together. Accordingly, researchers at the mammoth D0 detector (contact Greg Landsberg, landsberg@hep.brown.edu) have carefully looked for signs of monopoles (a pair of high-energy photons emerging from the debris of proton-antiproton collisions) amid the same data set used to discover top quarks. The results? No evidence for the monopole itself but new constraints on what its mass must be if it does exist: at least 600 GeV if the monopole is a spin-0 particle and 900 GeV if its spin is 1/2. This work will also help theorists grappling with the idea of dyons, hypothetical particles that have both electric and magnetic charge. (Abbott et al., upcoming article in Physical Review Letters.)