The fire had been extinguished earlier, but the rescuers had to wait until the tunnels were cleared of toxic fumes to go in.
So far, 32 miners have been rescued - the rest are said to be safe.
The accident happened at 0300 (0900 GMT) on Sunday in Esterhazy, in Saskatchewan province.
A representative for Mosaic Co, which operates the mine, had said the miners had food and water for up to 36 hours.
"We have had fires underground but none that we can remember that have been this tricky," Marshall Hamilton, head of human resources at the potash mine, told reporters.
