The decision of the Rugby Football League (RFL) to fine Castleford STG40,000 ($A71,000) for homophobic chants aimed at Crusaders winger Gareth Thomas has been labelled as "historic" by the player's agent.

The incident took place during a match at Castleford's home ground in a Super League fixture on March 26, Thomas's second match in the 13-man game.

Former Wales rugby union captain Thomas announced he was gay in December before switching codes to play in rugby league.

"Castleford were found guilty of unacceptable behaviour, of breaching the RFL's respect policy, of misconduct by their supporters and of conduct prejudicial to the interests of the sport," an RFL statement said in announcing the fine on Tuesday.

Thomas' manager, Emanuele Palladino, said: "Today has been an historic day.

"Gareth Thomas is all about delivering positive messages and positive changes and, to that end, today was a huge step forward for Gareth and the world of sport.

"It is not about the fine, but rather recognition that this behaviour from a small section of the Castleford fans is not acceptable in 2010."

Half of the fine was suspended until the end of 2011.

But the RFL ordered Castleford to pay STG20,000 ($A35,500) pounds immediately and forfeit the STG20,000 suspended from the Stg40,000 they were fined last year after bottles were thrown on to the pitch and officials were threatened following a game with Catalans Dragons.

Castleford chief executive Richard Wright said the club was "shocked" by the verdict and would appeal.

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