Sea bass, Dicentrarchus labrax naturally infected with L. kroyeri were exposed to dosage of trichlorfon for 7 days: 0 (as control), 50 mg kg(-1). On days 0, 7, 14, 21, 28 and 35 of the experiment, fish were randomly sampled and weighed, and then parasites on the gills were counted. Comparisons were made to untreated control fish. Sea water temperature and salinity in the pens were measured as 16.0-16.5 degrees C and 35 ppt, respectively. When the study began, all of the examined fish were infected with L. kroyeri. There were no statistical differences (P > 0.05) in terms of mean number of L. kroyeri within each group, including the control. According to the IRR coefficients, 50 mg dose of trichlorfon reduce the males at 98.3%, females at 97.1% and the total 97.1% when compared to the control group. No mortality was found and adverse drug reactions or palatability problems were not associated with the treatments during the study.