This study compared the population biology of two co-existing species of ghost shrimps, Trypaea australiensis Dana 1952 and Biffarius arenosus (Poore 1975), over a two year period at Warneet and Crib Point in Western Port, Victoria, south-eastern Australia. Overall, the sex ratio in populations of T. australiensis varied considerably (male and female biases were found at different times) whereas the sex ratio of B. arenosus was generally 1:1 or female biased. A male biased sex ratio was found in the juvenile size class of populations of T. australiensis (both years) and B. arenosus(one year only). Both species reproduced in spring and summer in Western Port and juveniles appeared to recruit into the populations all year round. The embryo and clutch size of T. australiensis females was signifi cantly larger than B. arenosus, and a signifi cant relationship between female body size and clutch size (but not embryo size) was found for both species. Comparisons between this and other population studies of T. australiensis and B. arenosus were made to highlight any latitudinal variation in the reproduction and breeding biology of these species along the eastern coast of Australia.