The Erythrean invasive swimming crab Charybdis longicollis established dense populations in the Levantine Basin of the Mediterranean Sea in the mid 20th century. The crabs were subsequently parasitized by the rhizocephalan Heterosaccus dollfusi, itself an Erythrean alien. In May 2008, the crab populations were sampled at the southern (Israel) and northern (Turkey) limits of its introduced range. The prevalence of infection was 3 times as high, and multiple externae-bearing hosts more than 4 times as many, in Israeli waters as in Turkey. It seems that off the Israeli coast, the water temperature permits the synchronous ontogenetic development of both host and parasite, ensuring the availability of plentiful young, recently-molted, prospective hosts for infection by the short-lived parasite cypris. It is possible that the lower water temperature off Antalya (Turkey) may affect the timing of ontogenetic development of one species or the other, or increase the mortality of infected hosts, resulting in drastically reduced parasite prevalence.