Many crab fisheries target large males, which can cause their densities to differ greatly in areas where they are fished and areas where they are protected, such as inside marine reserves. The size and gender of crabs potentially affect their ability to compete for resources, with competition likely to favour larger male crabs,raising the potential for cascading effects on females or smaller crabs in reserves. The role of size and gender in structuring the outcomes of competitive interference interactions between mud crabs (Scylla serrata Forskal) was tested by placing crabs in tanks with a competitor of smaller, equal or greater size, and of the same or differing gender. The behaviour of crabs was recorded for 15 min, and the times spent feeding, attacking, threatening and avoiding were compared among the different size and gender combinations. Male and female crabs spent less time feeding and more time avoiding in the presence of larger male than larger female crabs. Female crabs spent more time avoiding male crabs of equal size than female crabs of equal size. Male crabs of equal size spent the most time attacking. The size and gender-dependent responses of mud crabs to competition may lead to quite unexpected effects inside marine reserves.