In the Common woodlouse Porcellio scaber (Crustacea: Isopoda: Oniscidea), experimental immune challenge did not induce the expression of pro-phenoloxidase that, in most other invertebrates studied thus far, can be activated into phenoloxidase via an activation cascade upon immune challenge. Instead, Porcellio hemocyanin proved to exhibit catecholoxidase activity upon activation. However, none of the activating factors known from other invertebrates other than SDS-treatment resulted in activation of hemocyanin into a functional phenoloxidase in vitro. The distinct characteristics of isopod hemocyanin are reflected by the quaternary structure of the hemocyanin dodecamers that differs from that of other crustacean hemocyanins in that the two hexamers share a common 3-fold rotation axis and have an angular offset of 60 degrees against each other. Accordingly, the sequence of Porcellio hemocyanin can be distinguished clearly from other crustacean hemocyanins and in a phylogenetic analysis forms a cluster with other isopod and amphipod hemocyanins. We propose a peracarid-type hemocyanin that may have evolved in response to its required multiple functions in respiration and immune response, while phenoloxidase sensu strictu is lacking.