With a significant increase in experimental sensitivity, we have observed an additional 281 $K_L^0\to \mu \mu$ events after a background subtraction of 19±6 events. Normalizing this sample to the simultaneous observation of the decay $K_L^0\to {\pi }^{+}{\pi }^{-}$, we obtain the branching ratio $B(K_L^0\to \mu \mu )=[7.6\mathrm{}\pm 0.5(\mathrm{stat})\pm 0.4(\mathrm{syst}\left)\right]\times {10}^{-9\mathrm{}}$. Combining these data with our previously published sample containing 87 events yields $B(K_L^0\to \mu \mu )=(7.0\mathrm{}\pm 0.5)\times {10}^{-9\mathrm{}}$.

• Received 28 February 1991

DOI:https://doi.org/10.1103/PhysRevD.44.R1