THE first temperature-sensitive paralytic mutant of Drosophila melanogaster, parat s, was isolated four years ago1–3. The mutation was mapped as a point at 53.9 on the X chromosome4. The present day stock of parat s moves normally at 22° C, but is paralysed within 10 s at 29.5° C. Paralysis is usually complete. The legs are held close to the body causing the fly to lie on its back or side. When returned to 22° C the fly recovers its mobility within 5 s. A much slower and less complete recovery occurs when the flies are left at 29.5° C (ref. 4). Only after 30 min are they able to walk and after 2 h a few flies can be induced to fly by shaking their container. The mutation has been shown to act in a regionally autonomous fashion within the head and thorax5.