In gill-net fisheries around the world, very large numbers of small cetaceans are killed each year as bycatch (IWC report 1994). If commercial fishing is to continue in areas where the incidental catch of cetaceans is significant then technical improvements to the method of fishing are required to mitigate the impact. Modifying fishing gear by the addition of acoustic alarms which signal the position of the net by the transmission of low level sounds was pioneered by Lien and his colleagues in Newfoundland and this technique successfully reduced baleen whale interactions with set fishing nets and traps. These simple devices, which were constructed by the fishermen, were also tested in a bottom set gillnet fishery in the Bay of Fundy where harbour porpoise mortality is high. Encouraging results from this Canadian research resulted in an improved device being developed in the USA for use in a subsequent Gulf of Maine study. The low frequency signals developed as baleen whale deterrents were also tested on captive harbour porpoises at Harderwijk in Holland where it became very evident that the frequencies used were inappropriate. The technology used in these early devices is electro- acoustically inefficient and the operating (battery) costs rather high. In Europe studies of a variety of potential acoustic deterrent devices were carried out by Loughborough University which included tests with a harbour porpoise of a wide variety of signal frequencies and waveforms. These sounds were synthesised digitally and the behaviour of a 9ee swimming animal, contained within a large floating net enclosure, were observed. The new generation of micro-controller based beacon-mode alarms developed at Loughborough University is discussed here. These devices synthesise the sounds shown to be most aversive to a porpoise and implement new features intended to minimise habituation rates and maximise battery life. A preliminary field test with wild harbour porpoises in Scotland during September 1996 showed that these devices induce a dramatic avoidance behaviour, displacing the animals in a short test to a range greater than 640 m. The design and engineering of this new technology device is discussed in the context of preparing them for a large-scale commercial fishery trial in Denmark.