  The positive--strangeness baryon resonance $\Theta^+$ is observed in
photoproduction of the $\rm nK^+K^0_s$ final state with the SAPHIR detector at
the Bonn ELectron Stretcher Accelerator ELSA. It is seen as a peak in the $\rm
nK^+$ invariant mass distribution with a $4.8\sigma$ confidence level. We find
a mass $\rm M_{\Theta^+} = 1540\pm 4\pm 2$ MeV and an upper limit of the width
$\rm \Gamma_{\Theta^+} < 25$ MeV at 90% c.l. From the absence of a signal in
the $\rm pK^+$ invariant mass distribution in $\rm\gamma p\to pK^+K^-$ at the
expected strength we conclude that the $\Theta^+$ must be isoscalar.
