Heineken Cup holders Leinster will face Bath, Glasgow and Montpellier in the pool stages of next season's competition.
Meanwhile, Aviva Premiership winners Saracens face a tough draw with Biarritz, Ospreys and Treviso in Pool 5.
The draw in full:
Pool 1: Munster, Northampton Saints, Scarlets, Castres
Pool 2: Cardiff Blues, London Irish, Edinburgh, Racing Metro
Pool 3: Leinster, Bath Rugby, Glasgow, Montpellier
Pool 4: Leicester, Clermont, Ulster, Aironi
Pool 5: Biarritz, Ospreys, Saracens, Benetton Treviso
Pool 6: Toulouse, Harlequins, Gloucester, Connacht