Temnocephalans are recognised as common associates of freshwater crustaceans in Australia. This study has investigated the observed associations between the species Euastacus sulcatus and the temnocephalans Temnohaswellia and Temnosewellia spp. The results of this study support our observations that: (1) small crayfish below ~20 mm OCL host none or very few Temnohaswellia worms, (2) crayfish above ~20 mm host many Temnohaswellia and Temnosewellia worms, plus Temnosewellia eggs, and (3) female crayfish, above ~40 mm OCL, carry significantly fewer worms and eggs than males of a similar size. Our results indicate that the numbers of temnocephalans attached to crayfish rise strongly with host OCL, and are best predicted by a measure of the host crayfish surface area, the square of OCL, suggesting that worm populations are limited by the space available on crayfish. We believe that fewer worms on larger female crayfish, than large males, may be attributable to reproductive activities, particularly female preening, and, in the case of Temnohaswellia worms, predation by pre-independent juvenile crayfish. Our results support the conclusion of a strong fidelity between E. sulcatus and their ectosymbiont temnocephalans, so it is likely that many generations of temnocephalans complete their entire life cycle on their crayfish hosts.