Contracaecum rudolphii Hartwich, 1964 is a nematode which causes major concerns to human and wildlife animal’s health. However, the population genetics of C. rudolphii has been poorly studied in Iraq. In order to gain a deeper understanding in the outline of the genetic diversity of the nematode C. rudolphii that were isolated from its host cormorant Phalacrocorax carbo (Linnaeus, 1758), in the middle areas of Iraq, twenty specimens of C. rudolphii adults were isolated from nine individuals of P. carbo. The first (ITS-1) internal transcribed spacers (ITS) of ribosomal DNA (rDNA) of C. rudolphii were amplified using conventional polymerase chain reaction (PCR); then, the amplicons were subjected to sequencing. Concatenation of ITS-1 (rDNA) sequences resulted in four unique genotypes that have not been previously recorded in Iraq. The present study showed that the most common genotype occurred in 85% of C. rudolphii, and in 88.9% of cormorants. Furthermore, the infrapopulation difference in the genotypes was fairly high, with an average of 1.3 ± 0.48 genotypes per host of those with ≥two nematodes. All the sequences of the current study were distributed into two different populations. The sequences of ITS-1 for the first population had the highest similarity to ITS-1 sequence of C. rudolphii B, while the sequences of ITS-1 for the second population had the highest similarity to ITS-1 sequence of C. rudolphii A. This study provides an insight about the genetic divergence of C. rudolphii among P. carbo in Iraq. As well, the results likely support the hypothesis that C. rudolphii represents a complex of at least two sibling species.