A method is provided for treating or preventing blackhead disease in birds by administering to birds, especially to poultry, such as turkeys, benzimidazoles having the structure ##STR1## wherein R.sup.1 is lower alkyl or phenyl-lower alkyl, and R.sup.2 is lower alkyl, cycloalkyl, cycloalkylalkyl, cycloalkenyl, cycloalkenylalkyl, phenyl, benzyl and phenylethyl dispersed in a non-toxic physiologically acceptable carrier.
