  From the 2002 data taking with a neutral kaon beam extracted from the
CERN-SPS, the NA48/1 experiment observed 97 $\Xi^{0}\rightarrow \Sigma^{+}
\mu^{-} \bar{\nu}_{\mu}$ candidates with a background contamination of $30.8
\pm 4.2$ events.
  From this sample, the BR($\Xi^{0}\rightarrow \Sigma^{+} \mu^{-}
\bar{\nu}_{\mu}$) is measured to be $(2.17 \pm 0.32_{\mathrm{stat}}\pm
0.17_{\mathrm{syst}})\times10^{-6}$.
