The genetic structure of Anisakis physeteris from the Mediterranean Sea has been analysed electrophoretically at 22 enzyme loci. The samples studied, although differing in the life-stage (larvae and adults), and in the host (the fishes Micromesistius poutassou and Trachurus trachurus, and the sperm whale Physeter macrocephalus) were genetically homogeneous. Of these loci 11 (Ldh, Sod, Np, Adk-2, Pgm-1, Est-1, Est-2, Acph-1, Acph-2, Lap-2 and Ca) were found to be monomorphic, while the other 11 (Sdh, Mdh, ldh, 6-Pgdh, G3pdh, Got, Adk-1, Pgm-2, Lap-1, Mpi and Gpi) showed from 2 to 7 alleles. The following values of genetic variability were estimated: He = 0·ll, P = 0·50, A = 1·95. Distinct alleles were found between A. physeteris and the A. simplex complex at 19 out of the 22 loci studied, and only few rare alleles were shared at the remaining 3 loci. The genetic divergence between A. physeteris and A. simplex A and B is therefore very high, the values of Nei’s index D being 7·384 and 6·443 respectively (I = 0·001 and 0·002). The assignation of A. physeteris and the A. simplex complex to two distinct subgenera, Skrjabinisakis and Anisakis, as proposed by Mosgovoy on a morphological basis, appears to be fully justified according to our genetic data.