ABSTRACT During screening laboratory mice (Mus musculus) at the Zoology Department of the College of Science, King Saud University, some tiny oxyurid nematodes were detected from the large intestine of these rodents. Worms were identified using morphological and morphometric description. Furthermore, DNA was extracted from worms and subjected to polymerase chain reaction to amplify 18S rDNA and ITS (ITS1, 5.8S and ITS2) regions. Worms detected from caecum and colon of mice were detected in 80% of mice investigated. Morphologically, worms showed elongated body with tapered anterior end and narrow posterior part with cuticular annulations. Male worms measured 0.71-1.12 (0.90) long and 0.01-0.12 (0.11) wide. Female worms measured 3.12-5.011 (4.30) long and 0.13-0.29 (0.17) wide. Esophagus followed by intestine which opens at the posterior end via anal opening in females and via cloacal opening in males. Males have a single spicule with a gubernaculum and an accessory hook. Females’ uteri were pack with eggs. Data from 18S rDNA revealed a sequence which was identical to Syphacia obvelata in NCBI GenBank. Similarly, sequences from ITS regions grouped with sequences from S. obvelata confirming the morphological identity of the worm. However, it showed 3 mutations at the ITS2 region from related sequences from S. obvelata at NCBI GenBank.