The main index ended 0.16 percent, or 9.31 points weaker at 5,735.68, its lowest level since March 20.
'The market is down on year-end settlements and book balancing,' said a stockbroker on condition of anonymity. 'High interest rates are also hurting sentiment.'
The financial year for some of the main listed companies ends on March 31.
Yields in T-bills rose for the fourth straight week at Monday's auction even though the central bank had kept its key policy rates unchanged for a third month in a row.,
Turnover was 2.04 billion rupees ($16.08 million), twice this year's daily average of 1.01 billion rupees.
Foreign investors were net sellers of 33.8 million rupees worth of shares, but they have been net buyers of 4.92 billion rupees worth of shares so far this year.
The rupee firmed slightly at 126.80/85 to the dollar, from Wednesday's close of 126.85/90, dealers said.
Both the stock and money markets are closed for a holiday on Friday.
($1 = 126.8750 Sri Lanka rupees)
(Reporting by Ranga Sirilal and Shihar Aneez; Editing by Ron Popeski)
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