The BBC understands that the UUP has selected Colin Crawford to replace Robin Swann as assembly member for North Antrim.
A selection process took place tonight after Mr Swann was elected to Parliament in the general election.
Mr Crawford is currently a councillor for Mid and East Antrim.
The co-option must be officially declared to the chief electoral officer before an MLA can be sworn in to the assembly.
Alliance, the TUV and Independent MP Alex Easton have yet to announce their replacements for the assembly, but there is a deadline for this to happen within seven days.
The co-option system is used to fill vacancies at Stormont.
The process most commonly takes place after an election, resignation or death.
Stormont’s elected politicians are allowed to run in Westminster elections when they are assembly members.
But it’s against the current rules to be both an assembly member and a Member of Parliament at the same time.
Once seats become vacant, the co-option process can take place, as laid out in Section B of the Northern Ireland (Elections) (Amendment) Order 2009.
Step one is for the party of the MLA to nominate a replacement within seven days.
The Chief Electoral Officer will then write to the nominee asking them to confirm within seven days that they are willing and able to take up the post.
In the case of independents, it’s up to them to choose their replacements.
