Sound is an effective channel for the transfer of information underwater. While it is known that fish and whales can use sound for communication, and as a cue to localise predators and prey, much less is known about sound production in invertebrates. Here we describe sounds produced by two of the most common marine crustacean zooplankton in the Northern hemisphere: Northern krill (Meganyctiphanes norvegica) and copepods (Calanus spp.). The recorded sounds were taxon-specific and within the hearing range of common planktivorous fish. We recorded similar sounds in the laboratory and in the field. In krill, the sound co-occurred with a tail flip, and the amplitude of the sound was correlated to the displacement distance of the animal, indicating a potential sound producing mechanism. Our findings highlight the possibility that zooplankton sounds could be used as a cue for their predators and for intraspecific communication.