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We examine extensions of the Standard Model (SM), basing our assumptions onwhat has already been observed; we don't consider anything fundamentallydifferent, such as grand unification or supersymmetry, which is not directlysuggested by the SM itself. We concentrate on the possibility of additional lowmass fermions (relative to the Planck mass) and search for combinations ofrepresentations which don't produce any gauge anomalies. Generalisations of theSM weak hypercharge quantisation rule are used to specify the weak hypercharge,modulo 2, for any given representation of the non-abelian part of the gaugegroup. Strong experimental constraints are put on our models, by using therenormalisation group equations to obtain upper limits on fermion masses and tocheck that there is no U(1) Landau pole below the Planck scale. Our mostpromising model contains a fourth generation of quarks without leptons and cansoon be tested experimentally.Comment: uuencoded 44 page LaTeX file plus 5 postscript figures, uses epsf.st

Extending the standard model using charge quantization rules