More than 2 million people in the Houston metro area are being told to boil their water before cooking, drinking, bathing or brushing their teeth.
People should boil water for three minutes before consuming or using it, officials said.
The boil order is the result of a power outage at a purification plant, according to Houston's mayor.
Mayor Sylvester Turner said Monday that the measure is precautionary. Officials do not believe the water was contaminated, but they are conducting tests to make sure it is safe.
Turner said he expects the boil order to be lifted by early Tuesday.
"I have asked for a complete diagnostic review of this system to verify and identify that it does not happen again," he said.
Several schools in the area were closed on Monday as a result of the boil notice.