Karunanidhi, who arrived here yesterday on a three-day visit, met Singh at his 7, Race Course Road residence and is believed to have asked the Prime Minister to tell Colombo not to resort to firing at fishermen who cross the International Maritime Border.
Karunanidhi also discussed the ensuing Assembly polls in Tamil Nadu and the seat-sharing mechanism between the DMK and Congress, DMK sources said.
He is also scheduled to meet Congress chief Sonia Gandhi.
The Chief Minister had yesterday told reporters that he would discuss the fishermen attack issue with the Prime Minister.
"I have been discussing the fishermen issue with Singh on several occasions," he had said yesterday indicating that he would take up the issue with the prime minister again.
The meeting comes at a time when Foreign Secretary Nirupama Rao is in Colombo to take up the issue with Sri Lanka.
Rao was asked by External Affairs Minister SM Krishna to travel to Colombo to discuss with the Sri Lankan government the killing of Indian fishermen.
Reflecting the political sensitivities in Tamil Nadu, India has already reacted very strongly to the incidents in which two Indian fishermen were killed in the span of one month, notwithstanding Sri Lankan government`s claim that its navy was not involved.