This study investigated defensive behavior by the microturbellarian Stenostomum sphagnetorum against the predatory ciliate Dileptus margaritifer. When S. sphagnetorum was attacked by D. margaritifer, the microturbellarian released a mass of mucous material from the site of attack and swam away, while D. margaritifer ingested the released material. If the attack was repeated, S. sphagnetorum was killed and eaten by the predator. Observations on the predator-prey interactions between these two organisms suggest that the discharge of mucous material by S. sphagnetorum is the main component of defensive behavior. The following findings support this hypothesis: 1) individuals of S. sphagnetorum artificially deprived of glandular secretions through lysozyme treatment were more vulnerable to D. margaritifer and were more sensitive to the predator's toxin than were untreated animals, while they were indistinguishable from intact S. sphagnetorum in external morphology and in the capacity to grow; 2) supernatant containing the D. margaritifer toxin induced in S. sphagnetorum the same behavioral reactions observed in actual encounters with D. margaritifer.