The 120,000 residents of Niort and the surrounding region have been able to take the bus for free since Friday.
Ticket sales only make up 10 percent of the Niort transport network's 15 million-euro budget, partly due to the fact that a number of large insurance companies are based there.
French businesses are obliged by law to pay transport authorities a contribution according to the size of their payroll.
Niort expects passenger numbers to rise 30 percent by September 2019 and plans to expand bike rental and car-sharing schemes.
Dunkirk's 200,000 people will be dispensed from paying fares next year.
There, too, tickets contribute only a small proportion - 4.5 million euros - to the network's 40-50 million-euro running costs.
The Channel port expects to double the number of passengers by 2020 and is to invest 65 million euros in modernising its transport system.
30 towns now have free transport
About 30 French public transport networks are now free, most of them around relatively small towns but run by both right-wing and left-wing local authorities.
The first to introduce free buses was Compiègne, near Paris, which took the step in 1975.
Deputy mayor responsible for transport, Nicolas Leday, who is a member of the mainstream right Republicans, told Le Monde newspaper that it had given a boost to the local economy, an experience repeated in other towns where easier travel has helped town centre businesses.
While supporters claim the measure has economic and environmental benefits, critics, who include consumers' and providers' groups, point out that what is not paid by passengers is paid by taxpayers and warn of difficulties for towns without large employers.
Ticket sales bring in 40 percent of its revenue at the moment the cost of a travel card went up by 2.2 euros in August,