The Democratic Alliance (DA) said it accepted losing Johannesburg’s mayoral chain to the African National Congress (ANC).
But interim leader John Steenhuisen said they would be pursuing rebel councillors who voted against the party.
The ANC’s Geoff Makhubo was elected to the job on Wednesday.
A tough-talking Steenhuisen described the actions of councillors who failed to toe the party line as nothing short of “treasonous”.
“Obviously, with the secret ballot, it makes it very difficult to identify who exactly was involved in the treasonous behaviour yesterday. There is an investigation, but the likelihood of being able to identify who those culprits are is going to be very difficult.”
Of the 268 councillors who voted on Wednesday, Makhubo secured 137 votes, while the DA’s candidate Funzela Ngobeni received 101.
Steenhuisen said the party would have to deal with councillors who sided with the ANC.
“The biggest obstacle to us getting into power will often lie with ill-discipline in our own ranks and if we’re serious about being the alternative in South Africa, then it requires discipline within our ranks.”