We report measurements of B meson decays to two kaons using 253fb-1 of data collected with the Belle detector at the KEKB energy-asymmetric e+e- collider. We find evidence for signals in B+→K̄0K+ and B0→K0K̄0 with significances of 3.0σ and 3.5σ, respectively. (Charge-conjugate modes are included.) The corresponding branching fractions are measured to be B(B+→K̄0K+)=(1.0±0.4±0.1)×10-6 and B(B0→K0K̄0)=(0.8±0.3±0.1)×10-6. These decay modes are examples of hadronic b→d transitions. No signal is observed in the decay B0→K+K-, and we set an upper limit of 3.7×10-7 at 90% confidence level.