After the market closed Tilak Karunaratne, chairman of the SEC, confirmed in comments to Reuters he was coming under "immense pressure to resign".
The Colombo Stock Exchange's main index fell 0.16 percent, or 7.77 points, to end at 4,846.45, its lowest since July 18.
Stock brokers and dealers said the move, if true would dampen an already underperforming bourse which has fallen 20.2 percent so far this year.
"You just can't change the SEC chairman as and when you wish," said a stockbroker on condition of anonymity. "If that happens investors will loses confidence further."
In December the previous head of the SEC resigned "to uphold her principles," barely a month after her deputy was removed amid broker complaints that tougher regulations were hurting stock market prices.
Monday's turnover was 163.6 million Sri Lanka rupees ($1.24 million), well below this year's daily average of 870 million rupees.
Foreign investors bought a net 21.9 million rupees worth of shares, extending the net foreign inflow this year to 26.19 billion rupees.
The rupee was a tad weaker at 131.95/97 per dollar from Friday's close of 131.90/95 on importer dollar demand, dealers said. ($1 = 131.9000 Sri Lanka rupees)
http://www.cnbc.com/id/48646522