This time, the Canadian Civil Liberties Association and Canadian Association of Elizabeth Fry Societies have filed a petition in Ontario Superior Court.
The petition seeks to challenge the constitutionality of isolation, which the groups call cruel and inhumane.
They say the practice is "fatally flawed."
Last week, the B.C. Civil Liberties Association and a group that speaks for male prisoners began a similar action in British Columbia.
The federal government has insisted prison authorities must have the option of placing inmates in solitary where needed.