Search for the decay K sup + r arrow. pi. sup +. nu. nu

An upper limit on the branching ratio for the decay {ital K}{sup +}{r arrow}{pi}{sup +}{nu}{bar {nu}} is set at 3.4{times}10{sup {minus}8} (90% C.L.). In addition, a 90%-C.L. upper limit of 6.4{times}10{sup {minus}9} is set on the branching ratio for decays of the form {ital K}{sup +}{r arrow}{pi}{sup +}{ital X}{sup 0}, where {ital X}{sup 0} is any massless, weakly interacting neutral particle. Limits are also set for the case where {ital M}{sub {ital X}{sup 0}}{gt}0.