  Using a large sample of photoproduced charm mesons from the FOCUS experiment
at Fermilab (FNAL-E831), we observe the decay $D^0\to K^+\pi^-$ with a signal
yield of $149\pm31$ events compared to a similarly cut sample consisting of
$36760 \pm 195$ $D^0\to K^-\pi^+$ events. We use the observed ratio of $D^0\to
K^+\pi^-$ to $D^0\to K^-\pi^+$ $(0.404\pm 0.085\pm 0.025)%$ to obtain a
relationship between the $D^0$ mixing and doubly Cabibbo suppressed decay
parameters.
