Pompey maintained their lead at the top of the Division One table with a demolition of Derby at Fratton Park.
Paul Merson tapped home an early opener after his initial effort had come back off the crossbar, while Yakubu and Matt Taylor soon extended the advantage.
Frustrated former Blues boss John Gregory was sent from the County dug-out, but his side responded after the break, through Lee Morris and Georgi Kinkladze.
Any hopes of a comeback were quickly dashed, however, as Svetoslav Todorov's late brace sandwiched a second goal of the afternoon from Yakubu.