The last decade has seen the emergence of two new developments in behavioural ecology: the discovery of animal personalities and a new approach to the analyses of animal behaviour, biological markets. Although both regard individual differences in behaviour as adaptive in their own right, these two developments appear, at first glance, to be opposing approaches to analysing animal behaviour. Personalities consist of suites of behaviours that are correlated across situations (e.g. some animals are consistently more or less aggressive across situations than other animals) and assumes that animals are limited in their response to their environment. By contrast, the biological market approach considers organisms as traders of commodities (e.g. food or grooming) where the exchange rates of commodities changes with time and is influenced by shifts in the abundance and demand for the commodity, as happens in human markets. Biological markets thus view animal behaviour as extremely plastic whereas the putative existence of animal personalities suggests that animal behaviour may be relatively inflexible. However, the two approaches may be more similar than heretofore realized and may be complementary rather than opposing. Here I briefly review each approach and show how animal personalities can arise from a biological market situation.