SUMMARYWoodall, P.F. 1984. Kleptoparasitism in Hardheads and Pacific Black Ducks, including size-related differences. Emu 84: 65–70.HardheadsAythya australis, diving for freshwater mussels in a lake on the Queensland University campus, were chased by other Hardheads and by Pacific Black DucksAnas superciliosa.Only Pacific Black Ducks and male Hardheads were successful pirates (9.8% of chases), fewer female Hardheads were pirates and they were never successful. More female Hardhead hosts than males dived during a chase and females lost fewer mussels to Pacific Black Ducks than did males. There was no simple relationship between the numbers of pirates and the probability of successful piracy in a chase. During the period of observation, there was a decline in the median time of chases, particularly of female Hardheads, and also a decline in successful piracy (20 to 5%). It is suggested that male and female (= smaller) Hardheads may form an Evolutionarily Stable Strategy of divers and pirates.