Inexpensive, easy to breed and handle, the zebrafish has become a valuable ally for researchers in recent years. At the Institut Pasteur in Paris, several teams are studying its brain, which has characteristics similar to those of human beings. Among its many applications, scientists are using the transparent brains of its larvae to inject tumour cells from cancer patients. Using a technique that makes blood vessels and neurons fluorescent, it is possible to track the movement of cancer cells in real time. IMAGES AND SOUNDBITES ARRANGED IN SEQUENCES N°33KL4UM