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At 0103 GMT, the main TAIEX index fell 0.34 percent, or 29.42 points to 8,720.13, after opening flat.
TSMC and UMC shed 1.1 percent and 0.35 percent respectively, after both posting a month on month decline in sales for a third consecutive month in June.
Quanta Computer Inc was off 0.86 percent. The company reported a 11.22 percent decline in June sales on Friday.
The Taiwan dollar was flat at T$28.799.
Foreign investors were net sellers on in the previoius session, bringing their total selling to T$2.08 billion this month.
Copyright Reuters, 2011