We sampled the green crabs at sites where it was introduced (West Coast USA, East Coast USA, Tasmania) and in sites where it is native (Europe).  Crabs achieved larger sizes in the introduced regions, suggesting that he crabs performed better ecologically.  A comparison of limb loss among regions indicated no difference in the rise of predation between introduced and native regions.  The presence of nemertean worms and parasitism by larval trematodes, acanthocephalans. Parasitic barnacles, on the other hand was substantially higher in Europe than in introduced regions. Trematodes and acanthocephalans were found in Europe and on the East Coast of the USA while nemerteans were found in Europe and on the West Coast of the USA.  Rhizocephalan barnacles were only found in Europe.  On the west coast, the nemertean worm transferred to the green crab from a native shore crab. We experimentally determined that this nemertean was capable of feeding on ovigerous green crabs in the field.  In modeling the effect of a parasitic castrator, we found a direct association between the prevalence of a parasitic castrator and the degree to which the host population is reduced. This association was only direct at larger spatial scales where host recruitment was effectively closed. The parasitic castrator was limited to host populations with a minimum density. In some cases, time lags in the system led to extinction of the host population. Addition of parasites, as might be used in a biological control program also led to extinction of the host population. This might be augmented with a fishery for the host.