Elections Ontario says up to 2.4 million voters in the province may have had their personal information compromised.
It says two memory sticks are missing with information about voters in up to 25 ridings, including Halton and Hamilton Centre.
Elections Ontario says the data includes names and birth dates, but no SIN numbers or drivers' licence information.
Elections Ontario head Greg Essensa said the Ontario Provincial Police have been called in to investigate the breach.
The two employees responsible for the USB's have been fired.