Measurement of the branching ratio for the decay K sub L sup 0 r arrow. mu. mu

Concurrent with our search for the decays {ital K}{sub {ital L}}{sup 0}{r arrow}{mu}e and {ital K}{sub {ital L}}{sup 0}{r arrow}ee, we have observed 87 {ital K}{sub {ital L}}{sup 0}{r arrow}{mu}{mu} events. Normalizing this sample to the simultaneous observation of the decay {ital K}{sub {ital L}}{sup 0}{r arrow}{pi}{sup +}{pi}{sup {minus}}, we obtain the branching ratio {ital B}({ital K}{sub {ital L}}{sup 0}{r arrow}{mu}{mu}) =(5.8{plus minus}0.6(stat){plus minus}0.4 (syst)) {times}10{sup {minus}9}.