Some Internet genres, in particular Weblogs and discussion fora, have a dubious reputation for giving voice to strongly polemical discourses or hate-speech. This paper investigates the use of dehumanizing metaphors, specifically parasite metaphors, in British debates about immigration. It compares the range of metaphors used in Blogs with that used in online fora and in mainstream newspaper coverage and concludes that despite substantial variation, they can be categorised into four main scenarios, of which one includes dehumanizing metaphors such as depictions of immigrants as parasites, leeches, or bloodsuckers. Whilst this kind of stigmatizing imagery occurs across the three different media genres, the samples also show significant quantitative and qualitative differences: dehumanizing metaphors occur most often and their potential for aggressive argumentation and polemics is exploited in more detail in Blogs than in the fora, and least in the mainstream press. It is then asked what cognitive import this differential usage has in view of a) the discourse histories of such metaphors and b) their most likely present-day semantic motivation. The paper concludes that while it is unlikely that present-day users have detailed knowledge of the etymological and conceptual histories of such metaphors, it is also improbable to assume a wholly “unconscious” or “automatic” use or reception in the respective community of practice, and that instead it is more likely that they are used with a high degree of “deliberateness” and a modicum of discourse-historical awareness.