City officials say coliform bacteria was found in localized areas of the Flint water system.
They say people affected by the warning should not drink water without boiling it first, nor should tap water be used to brush teeth, wash dishes, or prepare food.
People with compromised immune systems, infants, pregnant women and the elderly could be at increased risk of getting sick from contaminated water.
The presence of coliform may be a sign that there's a problem with the water treatment system or the distribution pipes.
From a press release issued Sunday morning, the Flint Water Department says these are the affected neighborhoods:
"This boil notice is limited to the area bordered by Dayton Street (north), DuPont Street (east) Flint River (south), and LaVelle Road to Flint River (west). The current boil water notice is being expanded to the north. This boil water notice is being expanded to the area bordered by Stewart Ave. (north), Saginaw (east) Hamilton Ave (south), and Clio Rd (west)."