The present report demonstrates that a medicated food containing quinine kills the skin-inhabiting trophozoite stage of Ichthyophthirius multifiliis in ornamental fish. Artificially infected swordtails. (Xiphophorus helleri), black mollies (Poecilia sphenops), and black neons (Hyphessobrycon herbertaxelrodi) were used in the trials. The fish were maintained in groups of 10 or 20 inside aquaria (20 or 60 l) at 25 degrees C. Ultrastructure investigations by means of transmission electron microscopy revealed clear deleterious effects of quinine on the trophozoite stages. Following the initial application the outer limiting membrane of the trophozoite was broken at places. Within the nephridial plasma the plasma bridges were broken in part. After 2 days of treatment the lumen of the alveolar sac became enlarged. The food vacuoles in treated trophozoites were more electron-dense than those in the untreated controls. Numerous lipid droplets were found close to the vacuoles. The degree of damage in the nephridial plasma was intensified. When feeding was prolonged for 3 or more days, all kinds of damage became more extensive as seen in the trophozoites after 1 or 2 days of treatment. In addition food vacuoles in the final stages of digestion were no longer detectable. In long-term feeding tests, when typical ornamental fish species were fed three times daily ad libitum with the medicated food over a 12-week period, the animals showed no adverse clinical symptom. From the toxicological as well as the ecological and economical point of view the feeding of flakes containing quinine has considerable advantages as compared with conventional bath treatment.