The present study was conducted to estimate the prevalence of Giardia lamblia infection in some regions of Baghdad city, the relationship of source of drinking water with the prevalence of this intestinal protozoal parasite had been evaluate, The study revealed that the total rate of G.lamblia infection in humans was 68/375 (18.13%) according to three diagnostic methods, there was significant relation (p≤0.01) between infectivity rate (25%) of this parasite and persons who consumed tap water as a source of drinking water as compared to other source of drinking water. RT-PCR tpi gene application was able to detected and genotyping G.lamblia to assemblage A and B, since mix infection of assemblage (A+B) are more prevalent in fecal samples than others, while in environmental samples the assemblage A, assemblage B showed in 1/4 (25%), 2/4(50%) respectively and mixed assemblage (A+B) showed in 1/4 (25%) of samples. K e y w o r d s Giardia lamblia,Genotyping, infectivity rate, RT-PCR tpi gene.