There were disruptions to bus services in Swansea on Friday as drivers at the city depot stage a four-hour walkout at 0900 BST in a row over pay.
Talks between the union and company broke down this week
Talks between First Cymru and union officials broke down this week.
Drivers in Swansea are pressing for a pay package that would see their wages rise by almost 10%.
The company said buses from the Swansea depot were affected but the park-and-ride service and school contract services ran as normal.
Union officials say members are prepared to take further action in the future.
Friday's drivers' strike is the second to hit Swansea this year.
The Transport and General Workers Union (TGWU) had previously agreed a revised pay package which it recommended to members but was rejected.
Talks continued but the conciliation service ACAS withdrew from discussions earlier this week.
Regional organiser for the TGWU said the timing of the strike was designed to cause a minimum of inconvenience to passengers.
"Our members have indicated they are prepared to take further industrial action in the future," he added.
First Cymru said a substantial number of services continued to operate in Swansea including the Cardiff Shuttle, school and private hire buses, park and ride and National Express.
"First Cymru remains committed to resolving the dispute in a manner that will ensure the future employment prospects of all its employees in a viable business," said a spokeswoman.