Forward Ruben Castro netted in each half for the visitors in a scrappy game at the Romareda as Zaragoza remained rooted to the bottom of the standings with just 15 points from 23 matches, five behind 19th-placed Sporting Gijon.
Despite enjoying the lion's share of possession, the Aragonese club were unable to build on a 2-0 success at Espanyol in their last league outing, which was their first win in 15 games since mid-October.
They have been unsettled by financial problems and protests by supporters against their unpopular president and their victory against Arsenal to win the Cup Winners' Cup in 1995 is becoming an ever-more distant memory.
Sevilla-based Betis, who climbed to 12th on 29 points, took the lead four minutes before half-time when Castro stroked a superb half volley into the top corner from the edge of the penalty area.
The 30-year-old Canary Islander grabbed his second in the 68th minute when he followed up a Jorge Molina effort to slide the ball into the empty net.
Zaragoza fans began to drain away from the already half-empty stadium well before the final whistle and those left in the stands greeted the end of the game with disgruntled whistling.
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