A search for a scalar top and bottom quarks has been performed 
using a total data sample
at centre-of-mass energies of 192--209~GeV 
collected with the OPAL detector at LEP.
No evidence for scalar top or bottom quarks was observed. Limits have
been set on the scalar top mass in scenarios where the mixing angle 
between the supersymmetric partners 
of the left- and right-handed states of the top quark is zero, and when
the scalar top quark decouples from the $\rm Z^0$ boson.
A mass limit on the light scalar bottom quark 
has also been obtained.