  Searches for a scalar top quark and a scalar bottom quark have been performed
using a data sample of 182 pb-1 at a centre-of-mass energy of 189 GeV collected
with the OPAL detector at LEP. No evidence for a signal was found. The 95%
confidence level lower limit on the scalar top quark mass is 90.3 GeV if the
mixing angle between the supersymmetric partners of the left- and right-handed
states of the top quark is zero. In the worst case, when the scalar top quark
decouples from the Z boson, the lower limit is 87.2 GeV. These limits were
obtained assuming that the scalar top quark decays into a charm quark and the
lightest neutralino, and that the mass difference between the scalar top quark
and the lightest neutralino is larger than 10 GeV. The complementary decay mode
of the scalar top quark decaying into a bottom quark, a charged lepton and a
scalar neutrino has also been studied. From a search for the scalar bottom
quark, a mass limit of 88.6 GeV was obtained if the mass difference between the
scalar bottom quark and the lightest neutralino is larger than 7 GeV. These
limits significantly improve the previous OPAL limits.
