The genetic polymorphism of red cell adenosine deaminase (ADA) was investigated in five population groups in Israel: Ashkenazi Jews, non-Ashkenazi Jews from Iraq, Yemen, North Africa, and Arabs. The distribution of the ADA1 and ADA2 genes was found to be heterogeneous. The highest frequency of the ADA2 gene was observed among the Iraqi Jews (0.1494) and the lowest among North African Jews (0.0907). In spite of this heterogeneity, all the five communities shared a common trait, namely, significantly higher frequencies of the ADA2 gene than those observed in most populations from Western or Northern Europe. The trait of high frequency of the ADA2 gene had been described also in Mediterranean populations (Greece, Italy), Asiatic Indians and Filipinos.