The 52-year-old Seselj, who has been in the detention centre of the Hague-based UN tribunal since 2003, excused his non-attendance with his fatigue caused by his two-week hunger strike. Seselj is only drinking water.
His repeatedly-delayed trial had been due to start on Monday after the procedural hearing, but the registry's spokesman was quoted by the Reuters agency as saying in court: "This morning I was informed...that Mr Seselj does not want to attend these proceedings."
The presiding judge of the trial chamber, Alphons Orie, said Seselj had not submitted any written submissions on whether he should continue acting as his own defense council.
By his refusal to attend Monday's session, the court "understands this as a waiver of his right to be heard," Orie said.
In that case this Serb ultranationalist, a former political science lecturer, will be represented by a stand-by defence counsel, which had been assigned to him earlier due to his disruptive behaviour in court.
In 2003 when he surrendered to the UN tribunal, Seselj pleaded not guilty to charges of ethnic cleansing of one third of Croatia's territory and a larger part of Bosnia-Herzegovina with the aim of annexing them to the so-called Greater Serbia.
In his speeches and participation in public rallies, Seselj openly advocated the idea of establishing the Greater Serbia.