Residents of St. Hubert and St. Bruno de Montarville are being advised to boil their water.
Routine tests on the reservoir used by those neighbourhoods turned up the presence of e. coli bacteria, which can be killed by heat.
People should boil their water for one minute before using it for drinking, brushing their teeth, washing food or cooking.
They can also use bottled water.
The city of Longueuil says untreated water can be used for washing dishes in hot water, washing clothes, or taking showers.
Residents with questions can call the city of Longueuil at 450-463-7000.
