Spectrum　pooling　in　cognitive　radio　systems　is　an　approach　to　manage　the　available　spectrum　bands　from　different　licensed　networks.　Most　previous　work　on　spectrum　pooling　concentrates　on　the　system　architecture　and　the　design　of　flexible-access　algorithms　and　schemes.　In　this　paper,　we　present　a　cooperative　scheme　for　internetwork　spectrum　sharing　among　multiple　secondary　systems,　which　takes　into　account　the　price　and　spectrum　efficiency　as　the　design　criteria.　Specifically,　the　spectrum-sharing　problem　is　formulated　as　a　stochastic　bandit　system;　thus,　the　optimal　spectrum-sharing　scheme　is　simply　allocating　the　new　available　band　to　the　secondary　network　with　the　lowest　index.　Extensive　simulation　examples　illustrate　that　the　proposed　scheme　significantly　improves　the　performance　compared　with　the　existing　scheme　that　ignores　optimal　spectrum　sharing.