Normal tentacles on polyps of the black coral Antipathes fiordensis are less than 2 mm long and display well-defined, wart-like structures, the centers of which are marked by both flagella and microvilli. Both of these microappendages are characteristic of spirocytes, the dominant type of cnidocyte in normal tentacles. Sweeper tentacles, up to 15 mm long, form in apparent response to an alcyonacean epibiont. The external surface of the sweeper tentacle lacks the well-defined, wart-like batteries of the normal tentacle, and exhibits a general reduction in the appearance of surface microappendages. Nonetheless, there is a greater number of cnidae per unit area. No spirocysts are found in these sweeper tentacles. Instead, the cnidom is composed entirely of microbasic b-mastigophores (MbMs). More than 99% of these are of a single type that are structurally different from the MbMs found as a minority of the cnidae in normal tentacles. Changes in sweeper tentacle cnidae are compared with those occurring in modified tentacles of other anthozoans.