In fish resource assessment, it is very important to know about the behaviour and form of fish schools. This paper describes the three-dimensional (3D) morphology and internal structure of pelagic schools observed using vertical-scanning multibeam sonar. The acoustic data were collected in waters off Venezuela, Senegal, and Mexico. The data were used to derive metrics of school location, density, shape and internal structure from a total of 668 schools: 257 from Mexico, 343 from Venezuela and 68 from Senegal. In general, school shapes were amoeboid-like (e.g. spheres or ellipsoids) and not simply geometric. Also, the fish were patchily-distributed, forming both nuclei (groups) and vacuoles (empty spaces) within the schools. The results support the hypothesis of ‘auto-organization of the fish inside schools’, meaning that the distribution of fish within a school arises from the elementary reaction of the individuals.