The overexploitation of certain fish species has meant that the fishing industry has directed its attention toward the use of underutilized species such as mackerel.  The effective utilization of mackerel largely depends on its fat content, and, therefore, the fishing industry requires analytical techniques to rapidly grade mackerel according to its composition.  We have developed a hand-held ultrasonic device that can be used to provide information about fish composition.  The composition is determined from measurements of the ultrasonic velocity of fish, either at a fixed temperature or over a range of temperatures.  A series of empirical equations were developed to relate ultrasonic velocity measurements to fish composition.  We demonstrated that the compositions determined using the ultrasonic velocity technique were in good agreement with those determined by official methods for a number of fish species including cod, mackerel, salmon, and tuna.  The technique is now at the stage where a commercial instrument manufacturer could develop it, providing there is sufficient interest and a big enough market in the fishing industry.