Most drugs and xenobiotics induce the expression of cytochrome P450 (CYP) enzymes, which reduce the bioavailability of the inducer and/or co-administered drugs. Therefore, evaluation of new drug candidates for their effect on CYP expression is an essential step in drug development. The available methods for this purpose are expensive and not amenable to high-throughput screening. We developed a fluorescence-based in vivo assay using transgenic Caenorhabditis elegans worms that express the green fluorescent protein (GFP) under the control of various CYP promoters. Using this assay, we found striking similarities between the worm CYPs and their human orthologs in their response to treatment with various drugs. For example, the antibiotic rifampicin, one of the strongest inducers of the human gene CYP3A4, was the strongest inducer of the worm ortholog CYP13A7. Since worms can be easily grown in liquid medium in microtitre plates, the assay described in this paper is suitable for the screening of a large number of potential lead compounds in the drug discovery process.