We attempted to establish a system for sensing chemical substances using gene expression patterns in Caenorhabditis elegans (C. elegans). The target chemical substances used in this study were steroid hormones: estradiol and testosterone. These hormones are known to act as endocrine disruptors when the substances which come from outside are received by biological systems as an external (xenobiotic) factor. The effects of these substances on expression patterns were measured using the cDNA microarray. The hierarchical clustering method was applied to analyse the response patterns. They were classified according to the similarity of their responses. We found that each cluster, which was a group of similar response patterns, corresponds to one kind of chemical substance. This means that the C. elegans cDNA microarray can be utilized as a chemical sensing system.