This protocol describes a procedure for screening small molecules for bioactivity and a genetic approach to target identification using the nematode Caenorhabditis elegans as a model system. Libraries of small molecules are screened in 24-well plates that contain a solid agar substrate. On top of the agar mixture, one small-molecule species is deposited into each well, along with worm food (E. coli), and two third-stage or fourth-stage larval worms using a COPAS (Complex Object Parametric Analyzer and Sorter) Biosort. Three to five days later the plates are screened for phenotype. Images of the wells are acquired and archived using a HiDI 2100 automated imaging system (Elegenics). Up to 2,400 chemicals can be screened per week. To identify the predicted protein target of a bioactive molecule, wild-type worms are mutagenized using ethylmethanesulfonate (EMS). Progeny are screened for individuals resistant to the molecules effects. The candidate mutant target that confers resistance is then identified. Target identification might take months.