SAN FRANCISCO, June 11 (UPI) -- Twitter CEO Dick Costolo resigned from his position amid months of slumping share prices, the company announced Thursday.
Costolo will step down effective July 1. Jack Dorsey, co-founder and chairman of the board, will serve as interim CEO while the board searches for a permanent replacement.
Twitter said Costolo will continue to serve on its board of directors.
"I am tremendously proud of the Twitter team and all that the team has accomplished together during my six years with the company," Costolo said of his departure. "There is no one better than Jack Dorsey to lead Twitter during this transition. He has a profound understanding of the product and Twitter's mission in the world, as well as a great relationship with Twitter's leadership team. I am deeply appreciative of the confidence the board, the management team and the employees have placed in me over the years, and I look forward to supporting Twitter however I can going forward."
Twitter did not comment on the reason for Costolo's resignation, but there have been calls for him to step down for months. The company's share price has regularly dropped since reaching an all-time high of $69 in January.
When Twitter's quarterly earnings were leaked early in April, the company's share price dropped 26 percent.
Twitter co-founder Biz Stone told International Business Times on Wednesday that Costolo didn't take calls for his resignation seriously.
"He weathers it well," Stone said. "Dick's got a thick skin, and he used to be an improv comedian, so it's like water off a duck's back with him. He just deals with it very well. He's very funny about it. He jokes about it, it doesn't seem to bother him."