For the first time homostructural zinc-oxide (ZnO)-based p-n juctions are successfully fabricated. As-doped ZnO films are used for the p-type sides and Al-doped ZnO films for the n-type sides of p-n junctions. ZnO films have been deposited on p-type GaAs substrates by pulsed laser ablation in an ambient gas of oxygen to reduce oxygen vacancies. The ambient gas pressure was 40mTorr and the substrate temperature was 350-450 degreesC. The current-to-voltage (I-V) curves of the ZnO films very clearly show the characteristics of p-n junctions. This is the first demonstration of a p-n junction fabricated entirely with ZnO.