Bath scrum-half Peter Stringer said he was "disgusted" after apparently being spat at by opposite number Jerome Fillol in his side's 36-20 loss to Stade Francais in the European Challenge Cup.
Television pictures appeared to show Fillol spitting in Stringer's face from close range after an exchange of views between the pair in the 25th minute of Saturday's game at The Rec.
The independent citing officer, Irishman Eugene Ryan, has 50 hours from the time of kick-off to decide whether or not to cite Fillol for the offence.
Speaking after the game, 35-year-old Stringer said that spitting at an opponent was "as low as you can probably go".
On Sunday, he wrote on Twitter: "Disgusted at what happened yesterday in the game. No place for it in rugby or any walk of life."
Brett Gosper, the chief executive of the International Rugby Board, tweeted: "Spitting on Stringer should be punished to the full extent of the law. #NotRugby."
According to the IRB's disciplinary code, the punishment for spitting is a ban with a duration of between four and 52 weeks, depending on the severity of the offence.