A search for unstable heavy fermions with the DELPHI detector at LEP is rep
orted. Sequential and non-canonical leptons, as well as excited leptons and
quarks, are considered. The data analysed correspond to an integrated lumi
nosity of about 48 pb(-1) at an e(+)e(-) centre-of-mass energy of 183 GeV a
nd about 20 pb(-1) equally shared between the centre-of-mass energies of 17
2 GeV and 161 GeV. The search for pair-produced new leptons establishes 95%
confidence level mass limits in the region between 70 GeV/c(2) and 90 GeV/
c(2), depending on the channel. The search for singly produced excited lept
ons and quarks establishes upper limits on the ratio of the coupling of the
excited fermion to its mass (lambda/mf(*)) as a function of the mass.