An official with the Union of Canadian Correctional Officers says Matsqui Institution remained under lockdown as of late Saturday night.
Gord Robertson says the inmate's injuries were "very serious" and he was airlifted to hospital.
He says the aggressor was moved out of the institution to another prison.
Correctional Service of Canada spokesman Jean-Paul Lorieau says there was an "incident" on Friday that prompted a lockdown.
Matsqui Institution is a medium-security federal prison located in Abbotsford, about 70 kilometres east of Vancouver.