The All Share Price Index edged up 0.10 percent (1.54 points) to end at 1,533.79 while the more liquid Milanka lost 1.
75 percent (28.57 points) to close at 1,606.78.
Turnover was 45 million rupees.
Sujeewa Pieris of Bartleet Mallory Stock Brokers said he believed Dialog Telekom fell as investor sentiment weakened with the entry of India's Bharti Airtel, adding more competition into an already tough market.
Dialog Telekom fell over 12 percent or 70 cents to 4.80 rupees with 427,000 shares done
"Airtel's entry probably impacted sentiment. Also, we expect Dialog's profits to be lower," Pieris said.
"There's no significant buying coming in even at these levels for Dialog."
Dialog Telekom, a unit of Telekom Malaysia, reported a loss in the September 2008 quarter amid a price war among existing operators ahead of Airtel's entry.
Airtel held a news conference Monday to announce the launch of its services.
The other big losers on the Colombo bourse Monday were the cable companies.
ACL Cables fell over nine percent or three rupees to 29 rupees on small volumes after its profit warning on Friday which said it had been hit with un-quantified losses on derivatives bought to hedge against changes in copper prices.
The firm said in a stock exchange filing that it expected "a significant decline in the company's profitability following exposure to hedging contracts" in copper, a key raw material in electrical and other cables.
Kelani Cables fell over nine percent or six rupees to 59 rupees also on small volumes while Sierra Cables fell eight percent or 10 cents to 1.10 rupees with 600,700 shares done.
Pieris said ACL Cables profit warning triggered the panic selling and probably brought down the share prices of the other listed cable makers although there was no indication they too had bought material at higher prices.
Until the news broke there had been interest from both institutions and small investors on ACL Cables.
Lanka Cement was the most actively traded stock Monday, rising 12.
50 percent or one rupee to nine rupees with 486,700 shares done
Peiris said investors were buying into the stock on speculation that Lanka Cement's defunct factory in northern Jaffna could be revived now that the army had seriously weakened the Tamil Tiger rebels.
Two Indian cement manufacturers and Holcim's local unit had shown interest in reviving the plant which sits on a rich deposit of limestone, the basic raw material for cement manufacture.
The plant had been closed because of the threat from Tamil Tigers who have now been driven away by the army and cornered in the island's north-eastern region.
"Investors may be thinking that investment will now come in as it might be viable to restart the plant," Pieris said.
Conglomerate John Keells Holdings gained almost six percent or three rupees to 54 rupees with 148,700 shares traded after a big shareholder was quoted in the Sunday Times newspaper as saying that he would not be selling anymore.
The share had been sliding in recent weeks as significant quantities were sold.