In District 28, voters are selecting a successor to Democrat Jenny Oropeza. She died in office just before last year's general election, which she won.
In District 17, voters are choosing a replacement for Republican George Runner, who was elected to the State Board of Equalization last fall.
Under the voter approved open primary system, candidates from all recognized political parties are on the same ballot.
If no candidate gets more than 50 percent of the vote, the top-two vote getters advance to a runoff election.