The European Champions Cup match between La Rochelle and Bath on Thursday became the third weekend tie to be cancelled due to Covid-19, organisers said.
The decision was taken after a Scarlets player, who faced the English side last week, returned a positive test.
"Bath have informed European Professional Club Rugby (EPCR) that the Scarlets player in question has been identified as a close contact of 12 of the Premiership club's matchday squad from the Round 1 fixture," said a statement.
"With a significant number of their senior players consequently self-isolating, in addition to reported injuries to several other squad members, Bath are unable to safely field a matchday 23 of the required standard in order to fulfil the Round 2 fixture.
"The Pool A match between La Rochelle and Bath which was scheduled for Saturday (19 December) at Stade Marcel-Deflandre is therefore cancelled."
On Wednesday, the games between Toulouse and champions Exeter and the tie featuring Glasgow against Lyon were also cancelled for similar reasons.