Behavioral Caenorhabditis elegans mutants are sought for the purposes of neurobiological research. Until now, large numbers of worms with neuronal defects have been obtained through mutagenesis techniques. However, the existing screening procedures are not only time-consuming and low-throughput, but also tedious and labor-intensive. Therefore, developing a rapid and convenient method to overcome these difficulties is necessary. The present study demonstrates for the first time a microdevice for the rapid screening of chemotaxis-defective mutants based on their chemotactic response. The microchip is capable of automatic introduction, local immobilization, and controllable generation of concentration gradients during the screening assays. With this device, six C. elegans behavioral assays can be performed using various attractants without requiring anesthetics for local capture, and ten mutants are effectively isolated from 10(4) mutagenized worms in 100 min. The microfluidics-based method is robust enough to sort the chemotaxis-defective worms with 91 % accuracy from a large population of wild type animals during a mutagenesis screen.