We describe a protocol for performing RNA interference (RNAi) screens in Caenorhabditis elegans in liquid culture in 96-well plates. The procedure allows a single researcher to set-up and score RNAi experiments at approximately 2,000 genes per day. By comparing RNAi phenotypes between wild-type worms and worms carrying a defined genetic mutation, we have used this protocol to identify synthetic lethal interactions between genes systematically. We also describe how the protocol can be adapted to target two genes simultaneously by combinatorial RNAi.