The invention relates to a device (10) and a method for automatically eliciting a thermal avoidance response in nematodes, particularly in order to test pharmaceutical substances. The device (10) has a bearing surface (12) on which sample containers (14) of nematodes (16) are disposed, a laser (18), a selective laser-beam interrupter (27) to regulate the length of the laser pulse, aft and a scanner (22) for projecting and focussing the laser beam and for controlling a deflection. A light source (28) illuminates the sample container (14) with light, which brings about a contrast in the nematodes in the container. An additional light source (29) can also illuminate the sample container (14) with light in order to excite fluorescence-labeled areas of the nematodes, causing them to fluoresce. A real-time image processing device connected to the camera (30) determines the respective positions of all areas relevant for laser irradiation of the nematodes and makes this information available to a controller (32). The beam path of laser (18) is directed sequentially to different areas of individual nematodes by the controller using the scanner (22).