Oikopleura dioica is the only recorded appendicularian in the Black Sea. During the last two decades major changes in the O. dioica population size as well as total zooplankton community structures were recorded when invasive ctenophore species appeared in the Black Sea ecosystem. The state of the O. dioica population, before the invasion of ctenophores Mnemiopsis leidyi and Beroe ovata, is reviewed. The effect of the invasion of these ctenophores on the total zooplankton and particularly on the O. dioica standing stock is summarized from our long-term data and published information. The abundance, biomass and species composition of zooplankton greatly decreased after the ctenophore Mnemiopsis leidyi invasion due to predation by M. leidyi. Specifically, the abundance of O. dioica declined at that period. But with increasing numbers of a new invader, the ctenophore Beroe ovata, a predator of M. leidyi, in 1999, the zooplankton community began to recover. The population density of Oikopleura dioica also gradually increased to a level that was within the range of its abundance before the M. leidyi outbreak and even higher due to increasing its prey bacteria, which was provoked by the mucus released by B. ovata.