The karyotypes of the two sibling species Parascaris equorum and P. univalens are illustrated, as well as their chromosome morphology. The chromosomes are characterized by heteropycnotic extremities and the absence of a well-defined centromerical zone. Furthermore in P. equorum one chromosome pair appears distinctly longer than the other, which would exclude the evolution of this species by a simple process of tetraploidization; this is also ruled out by the electrophoretic data. The electrophoretic study of 27 enzyme loci in the two species has shown: 1) only two of these loci (Pgm and Est-1) are polymorphic according to the 1% criterion; 2) 22 loci out of 27 have fixed alternative alleles in the two species (Aldo, Est-2, Est-3, G-6-pdh, G-3-pdh, $\alpha$-Gpdh, Idh-1, Idh-2, Ldh, Odh, 6-Pgdh, Sdh, Xdh, Hk-1, Adk-1, Adk-2, Phi, Tpi, Got-1, Got—2, Me and Sod-1); 3) the Pgm locus polymorphic in both species, presents the $Pgm^{1.00}$ allele with a frequency of 95,07% in P. univalens, whilst in P. equorum the most common allele is $Pgm^{1.10}$ with a frequency of 89.66; 4) the observed mean heterozygosity per locus is very low: 0.88 % for P. equorum and 1.52% for P. univalens; 5) out of the 2238 specimens electrophoretically examined 2092 were identified as P. univalens, 127 as P. equorum, while 19 (1 male and 18 females) were hybrids; the hybrids were heterozygous at all loci fixed for alternative alleles in P. univalens and P. equorum and presented both the alleles characteristic of the two species; 6) in spite of the presence of natural hybrids electrophoretic data show no gene flow between the two species; 7) the genetic distance (D) between the two species, calculated with the Nei index, is 1.939 (genetic identity (I) = 0.144); such a high genetic distance was never before recorded for sibling species; it indicates that P. equorum and P. univalens are the product of a very ancient process of speciation which has affected a large part of the genoma.