2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/45635Z-strings in the Weinberg-Salam model including fermions are unstable for all values of the parameters. The cause of this instability is the fermion vacuum energy in the Z-string background. Z-strings with non-zero fermion densities, however, may still be stable.9 pages, Latex, 1 figure (uuencoded), to appear in Proc. of PASCOS/HOPKINS 1995, held in Baltimore, MD, March 22-25, 1995High Energy Physics - PhenomenologyHigh Energy Physics - Theorytext