Book excerpt: This book is about the behaviour of teleosts, a well-defined, highly successful, taxonomic group of vertebrate animals sharing a common body plan and forming the vast majority of living bony fishes. There are weH over 22000 living species of teleosts, including nearly all those of importance in com mercial fisheries and aquaculture. Teleosts are represented injust about every conceivable aquatic environment from temporary desert pools to the deep ocean, from soda lakes to sub-zero Antarctic waters. Behaviour is the primary interface between these effective survival machines and their environment: behavioural plasticity is one of the keys to their success. The study of animal behaviour has undergone revolutionary changes in the past decade under the dual impact of behavioural ecology and sociobiology. The modern body of theory provides quantitatively testable and experi mentaHy accessible hypotheses. Much current work in animal behaviour has concentrated on birds and mammals, animals with ostensibly more complex structure, physiology and behavioural capacity, but there is a growing body of information about the behaviour of fishes. There is now increasing awareness that the same ecological and evolutionary rules govern teleost fish, and that their behaviour is not just a simplified version of that seen in birds and mammals. The details of fish behaviour intimately reflect unique and efficient adaptations to their three-dimensional aquatic environment.