The back fat thickness of 77 live pigs has been measured with an ultrasonic instrument. The method is simple to use and the ultrasonic waves do not harm the pigs. For measurement of a pig on one day, 95% of the mean ultrasonic readings are within ±13·2% of the mean measurements on the carcass.
Evidence for variations between the breeds examined or between sexes is doubtful. By taking repeated measurements on a large number of days, the accuracy of the mean readings for a pig would approach the limit inherent in the method. This limit is determined by small differences between pigs and is estimated to be within ±4·2% for 95% of pigs.