Six field isolates of E. tenella isolated from outbreaks of coccidiosis during the 4th yr of continual application of monensin in Czechoslovakia were evaluated regarding their sensitivity to monensin, lasalocid and arprinocid. Three floor-pen trials were performed with each isolate. The trials started at the age of 21 days and lasted up to 49 days of age of chickens. Three isolates proved slightly decreased sensitivity to monensin. The activity of lasalocid against the 3 isolates was in 2 cases better than that of monensin and in 1 case was the same. Arprinocid controlled all 6 isolates in all 3 trials performed. The course of coccidial infection differed considerably in the individual trials.