Abstract. Aspects of the life cycle of the peritrich ciliate Zoothamnium intermedium, an epibiont on calanoid copepods in the Chesapeake Bay, were investigated using host and epibiont cultures. Experiments were designed to characterize the formation, survival, and attachment of free-swimming stages (telotrochs) and to assess whether telotrochs preferentially attach to primary (Acartia tonsa and Eurytemora affinis) or alternate hosts from the zooplankton community (the rotifer Brachionus plicatilis, barnacle nauplii, polychaete larvae, and a harpacticoid copepod). The results showed that telotroch formation started 2 h after the death of the host, with >90% of the zooids leaving the host carapace within 7 h. Formation of telotrochs was triggered only by the death of the host, failing to occur when the host was injured or unable to swim. Telotrochs failed to attach to non-living substrates and survived for only 14 h in the absence of host organisms, suggesting that members of Z. intermedium are obligate epibionts. Attachment success decreased with telotroch age, indicating that colonization success in nature may strongly depend on the ability to find a suitable host in a short period of time. Individuals exhibited no preferences in colonizing juvenile or adult stages of A. tonsa or E. affinis. While telotrochs were able to colonize barnacle nauplii and the harpacticoid copepod in the absence of individuals of A. tonsa or E. affinis, they did not attach to the rotifers or polychaete larvae. Telotrochs preferentially colonized individuals of A. tonsa when in the presence of other non-calanoid host species.