They're the most erratic, inconsistent team in European rugby and, unfortunately for Ulster, they stand in their way to the Heineken Cup quarter-finals at Ravenhill this afternoon.
Chief among Ulster's difficulties in this potentially decisive game in Pool 4 will be to work out which Biarritz side has turned up.
Will it be the uncoordinated, disorganised outfit that makes regular appearances in the French Top 14, or the club that blew the Ulstermen away in south west France last autumn in the second half of their second Heineken pool match of the campaign?
That 35-15 win at Stade Aguilera, just after Biarritz's victory at Bath, seemed to have put the pool in the hands of the Basque club.
It would have if they'd done what everyone expected and thumped little Aironi in Italy. But, true to tradition, Biarritz blew it, losing 28-27.
"We were terrible that day, we showed no concentration or consistency at all," said former England full-back Ian Balshaw.
"I think we were concentrating so much on trying to score four tries for a bonus point, we forgot about actually winning the match.
"But we all know that sort of performance won't be any use in Ulster. They are too good a side for us to play like that again and hope to get anything out of the game."
The challenge is two-fold today for the home team.
Because Biarritz, despite their Italian catastrophe, are still three points ahead at the top of the pool, Ulster have to ensure they deny the Basques a losing bonus point.
That means they must not only win, but do so by a margin in excess of seven points. And it goes without saying that they have to prevent Biarritz scoring four tries.
That is not likely anyway, particularly with heavy rain and high winds forecast for the north today.
So, conditions may dictate a grim forward battle, the match being all but decided by the rival packs and goal kickers, Ian Humphreys and Dimitri Yachvili.
But if Ulster have potential problems to solve, so too do Biarritz. Their notorious inconsistency means not even they know what sort of a performance they're likely to pull out of the hat.
They blow hot and cold and their spirits and hopes soar and drop, like swallows in summer.
Their other difficulty is subduing the Ulster spirit. No-one knows Ravenhill like Ulstermen, especially on a day of foul weather.
The men from the Basque club won't have their mood improved by the Irish weather. And the chances are, they won't play the conditions as smartly or cleverly as their opponents.
But Ulster coach Brian McLaughlin knows the stakes are high and has demanded a commensurate performance.
"There's no doubt that as far as Europe is concerned it's a defining moment for us," he said this week.
"We want to make sure we put in a quality performance which gets us the win so that qualification for the quarter-finals is still very much in our hands."
McLaughlin also picked up on a crucial factor. His team matched Biarritz well for 40 minutes in the first match, but must now double that effort to have any hope of success this time.
They will also have to do without injured South African No 8 Robbie Diack, a notable loss.
I suspect McLaughlin knows exactly what his men must do this afternoon. They must get in Biarritz's faces from the first whistle and stay there the whole game.
Pressurising the erratic Basques and sustaining that pressure is likely to be key to today's outcome.
What happens up front will be crucial, but the battle between respective half-backs Ruan Pienaar and Ian Humphreys, against Dimitri Yachvili and Damien Traille for Biarritz, will also be vital.
By choosing Traille, Biarritz are virtually conceding they plan to kick long and often, hoping to pen Ulster inside their own '22.'
Then they'll hope indiscipline offers Yachvili goal-kicking opportunities.
But Ulster will probably be thinking along similar lines themselves, using the wind adroitly and hoping Biarritz's discipline errs.
Four tries for anyone? I think not.
ULSTER -- A D'Arcy; A Trimble, N Spence, P Wallace, S Danielli; I Humphreys, R Pienaar; T Court, R Best (capt), BJ Botha, J Muller, D Tuohy, S Ferris, W Faloon, P Wannenburg
Reps: N Brady, B Young, D Fitzpatrick, T Barker, C Henry, P Marshall, I Whitten, D McIlwaine
Biarritz -- D Haylett-Petty; T Ngwenya, M Bosch, L Tranier, I Balshaw; D Traille, D Yachvili; E Coetzee, B August, S Marconnet; J Thion,E Lund; M Lund, I Harinordoquy, R Lakafia.
Reps: R Terrain, Y Watremez, A Barozzi, P Taele, S Vahafolau, J Peyrelongue, M Bond, I Bolakoro.
REF -- N Owens (WRU)
Ulster v Biarritz,
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