On Thursday, the Alberta Labour Relations Board agreed with the union that the Zoo Society and the City of Calgary constitute a common employer.
The Zoo Society had argued that after 42 years of using city employees who belong to CUPE at the zoo, the society should be able to terminate the arrangement and opt out of collective agreement provisions.
But the board ruled the zoo could not replace union workers with others earning lower wages.
Union president Don Monroe says now that the labour board decision is behind them, he hopes the union, the Zoo, and city officials can work together.
The City of Calgary did not take a position in the proceedings.
The Zoo Society says it sees the decision as a positive step forward and says they are looking forward to working with the city.