Postal workers are on strike in Calgary, Medicine Hat, Lethbridge and Edmonton on Monday.

It's part of a rolling, rotating strike strategy by the Canadian Union of Postal Workers, who have been without contracts for almost a year.

It means those cities will be without service for 24 hours, starting from 8 a.m. on Monday.

Negotiations between the union and the government began in November 2017. Mediated talks started in January.

The collective agreement for the rural and suburban letter carriers, with about 8,000 members, expired Dec. 31, 2017, while the collective agreement for the urban postal operations unit, which has 42,000 members, expired Jan. 31, 2018.

Monday marks the second time Calgary postal workers have walked off the job since the rotating strikes started on Oct. 22.

The union says it's pushing for improved job security, an end to forced overtime and better health and safety measures.