The invention describes the use of a microbial preparation of one or more bacteria,for example of the genus Bacillus in the manufacture of a formulation as a medicamentfor the treatment and prevention of infestation of sucking lice from the orderPhthiraptera, Anoplura, Rhynchophthirina and/or Pediculidae. The medicamentcan be applied to plumage, pelage or hair to control sucking lice.