Gill nets are effective for collecting samples of many fish species. These nets may be highly selective in their catch, depending on the mesh size or sizes used and on the size distribution and body shape of the fish in the population. Early studies related mesh selectivity to length or, in a few instances, to length and weight. Later studies showed that the selectivity of gill nets was related more closely to the girth of the fish than to the length, weight, or a combination of these. The girthometer presented here was designed for rapid and accurate measurement of the fish girth. Both speed and accuracy are important when very large numbers of fish are sampled for mesh-selectivity studies. It is also important that a girthometer be sturdy and rigid enough to provide accurate measurements in spite of rough handling and difficult working conditions.