Twitter CEO Dick Costolo [Photo: agencies]

Twitter CEO Dick Costolo is stepping down as of July 1st.

Twitter co-founder CEO Jack Dorsey will take over on an interim basis.

Costolo has told the company's investors he has been considering the move since last year.

There has been speculation Twitter's head of revenue and partnerships, Adam Bain, may be tapped to be the next CEO.

News of Costolo's plans to quit actually sent Twitter shares up some 7-percent in after-hours trading before leveling out during the Thursday session.