Previous observations on the mating behaviour of the southern calamary, Sepioteuthis australis Quoy & Gaimard 1833, revealed the potential for multiple paternities within egg strands, which contained up to eight eggs each. Six egg masses were laid in captivity from a possible 14 female and 20 male parents. By using five microsatellite loci, 112 eggs from 35 randomly selected egg strands were assigned sires using the potential fathers’ genotypes. Seven egg strands containing 22 eggs were also collected from the field to test that squid in captivity were apportioning eggs in natural ways, and that multiple paternity within egg strands was not an artefact of captivity. Ninety-seven per cent of strands laid in captivity showed multiple paternities. Similarly, multiple fathers were also evident in the field-laid egg strands. The maximum number of sires found within a single egg strand was four; however, three sires were more common. This is the first account of multiple paternities within egg strands laid by Sepioteuthis. It is likely that females are ‘spermatophore-limited’ when egg laying, and are promiscuous to ensure a ready supply of these sperm packets. Moreover, genetic diversity in a brood is increased by this mating strategy.