A jellyfish surrounds a plastic fragment, merging the synthetic material with its body; a water agency poster warns of dangerous plastic bottle ‘fish’ in the Mediterranean; marine organisms take shelter on and under synthetic materials. These are the denizens of a growing realm marine ecologists call the ‘plastisphere’, where sea life and plastics meet. Building upon multispecies ethnography, science and technology studies interrogations of nature/culture divides and the practical work of classification, this article explores the indeterminacy – the very plasticity (...) – of the category of ‘species’ as it is engaged in seriousness and irony, with living and nonliving bodies. First, I draw on participant observation at a nonprofit marine institute laboratory in California to trace the travels of plastic-creatures through attempts to disentangle them in the pursuit of scientific knowledge. Here volunteers sort tiny plastic bits from animal ones under the microscope, enacting material boundaries as they decide what gets counted as life (not plastic) and what does not (plastic). Second, I follow movements of plastic-creatures through public education campaigns, paying particular attention to assumptions about belonging and agency enacted with assumptions about whether and when plastic-species should or should not meet. I argue that the ‘danger’ of plastic relationships lurks not in associations but in the very categories used to know and live with forms of plastic and forms of life, in the kinds of belonging that emerge with kinds of materials, and in the failure to recognize the impossibility of their separation. (shrink)