Many people visiting Paris wonder whether canines or other pets are allowed in public transportation in the capital, including in metro trains, buses, and trams. Some tourists do opt to bring their pets overseas for longer stays, so this is likely to be an important question for them.
The Rules, in a Nutshell
In theory, only very small dogs transported in baskets or bags can legally be brought onto the Paris metro, and only under the condition that the dog will neither "inconvenience" nor "soil" other passengers.
The language is fuzzy, but I take this to mean you must "ensure they don't slobber on fellow passengers, or behave aggressively toward them". The same is true for Paris buses and tramways.
Furthermore, seeing-eye dogs and dogs specially trained to aid disabled travelers are allowed in public transportation irrespective of size, provided the traveler carries official identification for the dog proving his or her special status.
One exception to these simple rules does exist: on the Paris RER (suburban train network), you may bring larger dogs onto trains as long as they are leashed and muzzled. This is largely due to the fact that the commuter trains are, on average, more spacious. Bringing larger pets onto these trains is not perceived as an inconvenience in the same way.
There's Theory... and Then There's Practice
Despite these well-defined rules, in practice, Paris metro agents tend to be somewhat lenient with owners who bring larger dogs onto the metro, provided the dog is on a leash and has a muzzle.
I've often observed such dogs riding on trains, and as long as they're well-behaved and don't bother or frighten passengers, their presence isn't particularly bothersome.
Read related feature: Complete Guide to Public Transportation in Paris
This is admittedly all pretty arbitrary, however: you can be fined dozens of Euros for bringing a larger (especially unmuzzled) dog onto metro trains, and it's really up to the discretion of metro officials at the end of the day.
Your Safest Bet? Follow the Rules
At the end of the day, it's probably best to err on the side of caution and follow the local laws: only bring your dog along in public transportation if he or she is small enough to fit into a basket or a totebag. The same (rather hazy) rules apply on city buses and trams. Again, see above for a noteworthy exception relating to big dogs on RER commuter trains.
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What About Cats and Other Small Animals?
Cats and other small pets (hampsters, rats, ferrets, etc) can also be taken on metro trains, buses, and tramway cars in Paris provided they are placed in bags, baskets, or small carrying cases. I recommend the last option to ensure they don't escape, bother or injure other passengers.