Republicans picked up two of five open General Assembly seats previously held by Democrats in special elections held Feb. 26.
Bizzarro won 53% of the vote in Tuesday's special election, a 16-point swing to the GOP compared with Gerratana's reelection to a fifth term last November.
The Republican said his win was a rejection of Gov. Ned Lamont's proposals to expand sales taxes and implement highway tolls.
Zullo's win represented a 12-point swing to the Republican Party from the 2018 election in a district Democrats have dominated for three decades.
Democrats retained the other three open state House and Senate seats. All five seats were left vacant when Lamont appointed newly reelected senators and representatives to posts in his administration.
West Hartford Democrat Derek Slap will move from the state House to the Senate after winning a four-way race in the 5th Senate District, left vacant when incumbent Beth Bye stepped down in January.
New London city councilor Anthony Nolan held the 39th state House seat formerly held by Democrat Chris Soto, defeating Republican Kat Goulart, the Green Party's Mirna Martinez, and unaffiliated candidate Jason Catala.
A special election will be held April 16 to fill the vacancy in the 19th House District, which covers parts of Avon, Farmington, and West Hartford.