Chief financial officer Tim Morse will step in as interim CEO, and the company will search for a permanent leader to spearhead a battle in online advertising and content with rivals Google Inc and Facebook.
Shares in the company jumped 6 percent. They are scarcely higher than where they were when Bartz first took the reins in January 2009 with hopes of reviving stalled growth and competing with up-and-coming rivals.
On Tuesday, her efforts were abruptly halted after Bostock called with the bad news.
The turn of events surprised few Wall Street observers who had tracked a rising torrent of criticism and watched revenue growth falter and sputter out.
Some analysts said Bartz's departure signalled the company had run out of options after failing to dominate the advertising and content markets and handing over its search operations to Microsoft Corp .
That partnership, under which Microsoft handles search for Yahoo's websites and keeps a portion of ad revenue, appears to favor the software giant at Yahoo's expense.
Yahoo said that a newly-formed executive leadership council would help Morse in managing day-to-day operations as well as supporting "a comprehensive strategic review" to position the company for future growth.
"It's hard to say what direction they are going to head. What is the next step for Yahoo? They went down the road of search, they went down the road of media, becoming a content company, they went down the road of advertising," said YCMNet Advisors CEO Michael Yoshikami.
"I'm not sure where they go right now. One wonders if this means that they might be ripe for a takeover."
Bostock voiced his public support in June for the CEO, a lightning rod for criticism from Wall Street, and known for her tough attitude and salty language.
Bartz's ouster capped a decade-long fall from grace for a company whose shares traded at more than $125 in January 2000 during the dotcom bubble -- but now languishes at about a 10th of that level.
Yahoo is still one of the most popular destinations on the Internet but faces increasing competition from social networking service Facebook and from Google.
Bartz arrived at Yahoo in January 2009 after a solid showing at software giant Autodesk with high hopes of turning around Yahoo, after co-founder Jerry Yang was widely thought to have botched a $47.5 billion proposed takeover by Microsoft, rebuffing that advance as too low. Yahoo is worth about $16 billion today, with much of that ascribed to its slice of Alibaba.
The Internet company reported a slight decline in net revenue in the second quarter, as efforts to restructure its sales force caused disruptions.
Research firm eMarketer has projected that Facebook would overtake Yahoo this year to collect the biggest slice of online display advertising dollars in the United States.
The management and board also came under fire after the company's handling of its relationship with China's Alibaba Group, in which Yahoo owns a stake of roughly 40 percent.
The rocky relationship between the companies came to a head in May when it was revealed that Alibaba had abruptly handed Alipay -- one of Alibaba's crown jewels -- to a company controlled by Alibaba founder Jack Ma, apparently without Yahoo's knowledge.
"They are going to need a vision of what they want to be in the future. And I don't think investors really understood what that clear vision was," Yoshikami said.