Strzok replied that he was not under subpoena, but testifying voluntarily - and that he could not answer questions "related to the ongoing investigations being undertaken by the special counsel's office".
"I testify today with significant regret, recognizing that my texts have created confusion and caused pain for people I love", Strzok said.
Strzok has already spoken at length to the House Judiciary Committee behind closed doors; Democrats have demanded - without success - that the Republican-controlled committee release the transcripts.
House Republican leaders threatened to hold former FBI lawyer Lisa Page in contempt of Congress unless she agrees to testify by Friday morning about her role in the bureau's probes into Hillary Clinton's emails and President Donald Trump's suspected Russian Federation ties.
In one particularly testy exchange, Trey Gowdy of SC questioned Mr Strzok about messages in which he said "we'll stop" Mr Trump's election and that Democrat Hillary Clinton should win 100m votes to zero.
"That's a disgrace", Cicilline said.
Democratic Congressman Steve Cohen found himself in hot-water Thursday among America's courageous service men and women; bizarrely saying anti-Trump FBI agent Peter Strzok deserves a Purple Heart over his escalating scandal.
Conservative commentator Ann Coulter said in a tweet:"I hope lots of Americans are watching the Strzok hearings".
"Mr. Strzok, I don't know where to start", Cohen said as he started his allotted five minutes.
As far as Gowdy's actual question goes, it turned out Strzok's refusal to answer didn't even matter.
Donald Trump and Melania arrive at Theresa May's banquet amid protests
Johnson resigned as foreign secretary on Monday, in protest over May's proposal for a " soft " Brexit from the European Union. A three-course meal will be served to guests, who include a number of prominent British business leaders.
The Purple Heart typically is awarded to members of the USA military who are wounded or killed by enemies in war.
The hearing often descended into partisan bickering over points of parliamentary order and whether or not Strzok could be compelled to answer questions pertaining to ongoing FBI investigations.
Republican Representative Bob Goodlatte, who heads the House Judiciary Committee, threatened to hold Strzok in contempt at the end of the hearing if he did not answer Gowdy's question.
When Strzok declined to answer some questions on the Russian Federation probe, Goodlatte suggested Republicans might recess the hearing and hold him in contempt. Democrats objected and Goodlatte eventually let the hearing proceed. Originally, Fusion GPS was hired by Trump's Republican opponents to do research, but after he became the party's nominee Democrats began funding it.
Calling the hearing a "new low", he condemned the whole affair.
Goodlatte went on to say she is still under the subpoena after dubbing it "disturbing" when she refused to comply with it on Wednesday.
While reading one in which he used the f-word while talking about Trump, Strzok paused and asked how he should handle it, then finished.
Page told investigators that she and Strzok were having an affair, according to a Justice Department Inspector General's report about the Clinton email investigation, and that they used their work devices to hide that from their spouses.
"But the thought of exposing that information never crossed my mind", Strzok said. "And it won't be recaptured any time soon because of the damage you've done to the justice system". Special counsel Robert Mueller removed him from the investigation after it came to light he had sent text messages to Page insulting Trump and praising Clinton. "It's something culturally that doesn't occur", Strzok told lawmakers.
That's according to prepared remarks obtained by The Associated Press. In breaking his months-long silence, Strzok vigorously defended his handling of two hugely sensitive investigations in which he played a leading role: inquiries into Hillary Clinton's email use and possible coordination between the Trump campaign and Russian Federation.