Southern has had to cancel certain trains, while others are expected to be severally delayed, including routes to London Victoria.
Southern said: "The strike will have a significant effect on Southern services on the whole of the day affected, with no service on many routes and only a limited service between approximately 07.30 and 18.00 on others, though we will extend the services hours if we are able."
The strike began at midnight.
It added that it continues to urge RMT to "engage with us and avert these completely unnecessary strikes".
Commuters can download a map that shows the effect on a station by station basis by following this link.
And a live departure board can be found on this link.
Bikes will not be permitted on Southern, Gatwick Express or Thameslink trains (south of Blackfriars) on the dates affected by strike action.
Read more: Flights cancelled due to French strikes
Just days ago the RMT's general secretary Mick Cash said: "Our message to passengers is that our fight is your fight and that the battle for our jobs today is to protect your safety tomorrow.
"If we stand together we can defeat this company and its attacks on jobs, services and safety."
Some gathered to express their anger:
BBC South East (@bbcsoutheast) June 21, 2016
Commuters, however, were not impressed. In Brighton a small crowd assembled:
Further industrial action could lie ahead as the RMT's executive committee plans to meet after the latest strike and rule on whether further action should be taken.
In other news, yesterday the RMT drew attention to new parliamentary questions that showed government fines on Govia Thameslink Railway - which manages Southern - for poor performance are less than the £3.4m the company paid in bonuses and salary to just two of GTR's directors.