It was hypothesized that energetically efficient anguilliform swimming and axial elongation in fishes is favoured in the deep sea and predicted that the degree of elongation of the body form of fishes would increase with depth. An index of fish shape was derived from the relationship between length and mass. This was combined with data on abundance of c. 266 fish species from 389 research trawl tows made at depths of between 300 and 2030 m in the north-east Atlantic Ocean. The degree of elongation of the fish increased with depth to c. 1250 m before levelling off. The strength of this phenomenon varied between higher level taxa, being most apparent in the Gadiformes and Osmeriformes, and weak or absent in the Perciformes and Selachimorpha. The advantage of efficient elongate body forms may explain why certain taxa such as the grenadiers (Macrouridae) have dominated the deep sea, some have restricted depth ranges, e.g. the sharks, skates and rays, and others are almost entirely absent, e.g. the flatfishes (Pleuronectiformes).