The problem of the clogging up of sea water pipes by Mytilus edulis larvae is a very real problem. A study has been made of the ways to destroy these larvae, (1) by electricity (publication in preparation), (2) by ultrasound. This last technique has given interesting data, but exploitation of results on a large scale presents many difficulties. An interesting general phenomenon is to be underlined. For ultrasound treatments of short duration (10s), many larvae are simply stressed and recover later on. For longer treatments, there is actual dislocation of the larvae and instant mortality, this mortality increases with time.