The retired neurosurgeon has gained 10 percentage points in Iowa since the last Bloomberg/Des Moines Register poll in August, while Trump has dropped four points.
This is the second recent poll to find Carson unseating Trump as the frontrunner in Iowa. According to a Quinnipiac University poll released on Thursday, Carson led Trump by eight points among likely Republican Iowa caucusgoers.
Following Carson and Trump in the Bloomberg/Des Moines Register poll, 10 percent of likely caucusgoers supported Sen. Ted Cruz (R-TX) and nine percent backed Sen. Marco Rubio (R-FL).
Bloomberg/Des Moines Register surveyed 401 likely Republican Iowa caucusgoers via phone Oct. 16-19 with a margin of error plus or minus 4.9 percentage points.