The aggressive behaviour of female hamsters was studied while they were housed in large enclosures with males and in brief tests with males or females. Some females are not aggressive with any male, whereas others are very aggressive toward all males in both testing conditions. Females that are not aggressive toward intact males may be very aggressive toward castrated males or females. When the animals are housed together for long periods of time, males dominate only if they are much heavier. Male dominance takes a relatively long time to establish and often there is an equivocal period characterized by reversals of dominance. Female dominance is rapidly established. Unless the male is much heavier, the female determines the presence or absence of agonistic behaviour.