Dialog closed down 25 cents at 25.50.
Brokers said there were about 10 transactions of Dialog shares. Most of the buyers were foreign funds.
Telekom Malaysia has said it will not bring its shareholding in Dialog Telekom down to less than 80 percent.
Colombo-based telecom analysts viewed Telekom's decision as a move to appease overseas investors who have in the past complained about the lack of liquidity in the counter. The Colombo Stock Exchange said the deal drove turnover to over 6.6 billion rupees.
The previous highest was 4.8 billion rupees on September 19, 2003.
Overall, share prices fell again driven down by worries over high interest rates, the new company law and political uncertainty, brokers said.
The All Share Price Index closed down 32.02 points or 1.27 percent at 2,493.87 and the Milanka fell 22.85 points or 0.63 p