South Wales Fire and Rescue Service say they received 'multiple calls' of flooding in Treorchy whilst on strike last night.
Up to 30 homes were evacuated in the Rhondda Cynon Taf area and a rescue centre was set up to help residents.
Members of the Fire Brigades Union across England and Wales staged their sixth walk-out since September in a row over pensions.
The strike happened between 6pm and 10pm.
Assistant Chief Fire Officer Rod Hammerton said he was pleased with how South Wales Fire and Rescue Service had managed its services during the strike action.
“As expected, the number of Firefighters from South Wales Fire and Rescue Service who chose to strike was high, which resulted in a large reduction in the resources that we had at our disposal.
"As such, we were unable to provide the same high level of response to incidents as we normally would and as a consequence prioritised our response to those in most critical need."
Rod Hammerton also added that contingency plans were put in place in collaboration with South Wales Police and the ambulance service to "ensure that the necessary advice was available to ensure that the resources that the public needed were available as quickly as possible."
It is unclear whether there will be further action by firefighters.
Fire crews across Wales will strike for the fourth time tomorrow in an ongoing dispute over pensions.
Firefighters who are forced to retire before 60 as a result of declining fitness could now receive pensions of just over £9,000 a year as a result of the latest government proposals.
The government has, until recently, claimed that firefighters who work for 40 years would receive £19,000 a year.
Fire Brigades Union general secretary Matt Wrack said: "After 35 years of service - and paying at least £4,000 a year - firefighters could now receive just over £9,000 a year, or the sack, simply because fitness declines as they get older.
"Firefighters simply want an affordable and workable pension that reflects the job we do. But with employee costs going up, firefighters are being priced out making the scheme threatening its sustainability.
"We're keen to resolve this through negotiations, but the government is simply ignoring all the evidence we have submitted."
The strike will take place between 10am and 2pm on Wednesday. Firefighters are also voting in a ballot over additional forms of industrial action.
Firefighters across Mid and West Wales fear lives could be lost because of changes in working practices. It comes just a week after the Fire Brigades Union spoke of its concern about plans to replace some frontline fire engines with smaller vans.
Some firefighters say that other changes within the service are also putting lives at risk. They say changes have been introduced in order to save money. And they argue that's a step too far, a claim that the man in charge of the service rejects.