The seat was left open after the recent redistricting process left incumbent state Rep. Kyle Biedermann, R-Fredericksburg, living outside the new district boundaries.
As of 10:15 p.m., 34 of 34 precincts in Comal and 91 of 91 precincts in Hays had reported results.
Reports show Isaac leading with 50.64% of the vote, or 11,176 votes. Barron Casteel has 49.36%, or 10,892 votes.
Neither candidate received a majority of ballots cast during the March 1 primary.
The Republican winner of the primary election for the seat will advance to the general election in November. Only one Democrat, Justin Calhoun, filed for a place on the ballot in the race and will also advance to the Nov. 8 general election.
Election results are unofficial until they are canvassed and certified.
