Betis, which reserved six first-teamers for Tuesday's Champions League qualifier against Monaco, soon answered with two goals by the 30th minute.
Dani volleyed from Xisco Munoz's sweeping left-sided cross in the 24th, and the second arrived six minutes later when the striker took advantage of a poor clearance from defender Oleguer Presas to shoot home from close range.
With Ronaldinho demonstrating the skills which won him FIFA's World Player of the Year last December, Barcelona dominated the second half and should have leveled in the 66th minute when Eto'o shot tamely into Contreras' arms.
Barcelona right winger Ludovic Giuly was replaced by Sweden's Henrik Larsson in the 85th minute, but Betis held firm to gain a morale-boosting victory a week ahead of the new Spanish season.
Betis visits Valencia, while Barcelona plays at newly promoted Alaves in the opening round of Serie A football.
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