A 40-year-old male, who is a Shanghai resident, presented to the emergency department of Yangpu Hospital, School of Medicine, Tongji University in July 2018. The patient complained of watery diarrhea for 1 day. The patient had a travel history to Panama one month before the onset of the disease and had no history of drinking unboiled water or consuming uncooked meat. The routine blood cell count results were normal except for an increase in C-reactive protein of 16.17 mg/L (normal value 0-10 mg/L), which indicated an infection. Giardia trophozoites and cysts were found in stool specimens by microscopic examination. The IFA test results showed that G. lamblia trophozoites was recognized by the patient’s serum (1 ∶ 50), and there was strong fluorescence staining on the surface of G. lamblia. A 147 bp fragment of Giardia-specific 18S ribosomal RNA was amplified by PCR. The sequence of this fragment was 100% consistent with the sequence of G. lamblia assemblage A (GenBank login number: KY706490), and it was clustered on the same branch with G. lamblia assembly A in the phylogenetic tree constructed by the neighbour-joining method. Based on the patient’s clinical manifestations and relevant examination results, a G. lamblia infection diagnosis was made. The patient was treated with oral metronidazole at 20 mg/kg daily, divided into 3 doses and take for 7 days. Blood and fecal routine tests were re-examined two weeks later, and both results returned to normal. The patient’s condition was improved without recurrence of symptoms.