Air travel: The round-trip fare from Paris to Bayonne/Biarritz is about $200.
Currency: Exchange plenty of your traveler's checks into local currency before hitting the road. Because while each town of any size in the Pyrenees has a bank or a shop that converts currency, you have to find the perfect hour when they are open each day. And the currency exchange rates are not good. And some banks don't work with American dollars or traveler's cheques and are willing to only change either pesetas or francs. The guides don't take credit cards and most of the restaurants don't either.
Runoff/timing/seasons: May and June are the time of runoff in the Pyrenees, but you can always find a stream just finishing runoff or in pre-runoff.
Guides: In these Pyrenees, don't expect the same services you receive in the United States. You won't get lunch. You won't get flies supplied. You also don't pay as much, with most charging about $150 for one or two persons. Half-day rates are usually available. Many guides are knowledgeable, courteous, friendly, eager to please, excellent anglers and useful almost as much as tour guides as fishing guides.
Language barrier: Unless you speak French, expect communication problems. Some locals speak Spanish and fewer know English.
Gear: Any travel rod less than a 6-weight will be fine. If you fish the big rivers, like the Gave d'Pau and Gave d'Oloron, an 8- to 9-foot rod of 5- or 6-weight is ideal. For the smaller rivers, consider a 7- to 8˝-foot rod of 3- to 5-weight outfit. A 6X tippet can be used exclusively. Wade wet or wear lightweight waders, but make sure to have felt soles. It doesn't hurt to have a wading staff in these slippery streams.
Trip setup: The choices, even for a small section of the northern Pyrenees, are overwhelming, so much so, you may consider hiring a travel service.
The services of www.gourmetfly.com (or firstname.lastname@example.org) can be a big help, provide such detailed information as what roads to take, where to turn, how to find the hotel, the best restaurants in the town. Hotel rooms can be secured without putting down a deposit and fishing licenses can be waiting for you upon your arrival there.