A search for pair produced scalar fermions with couplings 
that violate $R$-parity has been performed 
using data samples corresponding to an integrated luminosity of
about 700~pb$^{-1}$ collected with the OPAL detector at LEP 
above the ${\mathrm Z}^{0}$ peak, at centre-of-mass energies 
up to $\sqrt{s}=$ 209~GeV.
An important consequence of $R$-parity breaking interactions is that 
the lightest supersymmetric particle is expected to be unstable. 
Searches for $R$-parity violating decays of charged sleptons, 
sneutrinos and stop quarks have been performed under the assumptions 
that the lightest supersymmetric particle decays promptly and 
that only one of the $R$-parity violating couplings is dominant 
for each of the decay modes considered. 
Such processes would yield multi-leptons, jets plus leptons or 
multi-jets, with or without missing energy, in the final state.
No significant excess of such events has been observed.
Limits on the production cross-sections of 
scalar fermions in $R$-parity violating scenarios are obtained. 
Mass exclusion regions are also presented in the framework
of the Constrained Minimal Supersymmetric Standard Model.