Isoenzyme profiles of four strains of Giardia lamblia and their infectivity to jirds. The infectivity to jirds (Meriones unguiculatus) and the cyst excretion pattern of a recently isolated strain of Giardia lamblia from Egypt, Strain CDC:1088:1 (EGY), were compared to those of three well-established strains. All five jirds inoculated orally with strain UNO:0487:1 (UNO) became infected and began excreting cysts 3-6 days post-infection (dpi); no cysts were detected between 8-12 dpi after which time cysts were produced through day 19. Four of the five jirds infected with Strain ATCC:30957 (WB) and three of the five jirds infected with strain CDC:0284:1 (VA) excreted cysts from 6-20 dpi and 6-22 dpi, respectively. One of five jirds inoculated with EGY excreted cysts on 8 dpi only. At necropsy, trophozoites were recovered from only three UNO-infected jirds but from all WB- and VA-infected jirds that excreted cysts. The one jird which excreted cysts of EGY was negative at necropsy, but EGY trophozoites were found in one non-patient jird. Isoelectric focusing indicated that these four strains of G. lamblia represented three zymodemes. WB and VA were assigned to one zymodeme, EGY to a second, and UNO, which shared common bands with both other zymodemes, to the third. Although the similarities and differences in infectivity and cyst excretion patterns appear to coincide with the zymodemes to which the strains can be assigned, further study is needed to examine the parasitologic behavior of these strains in relation to isoenzyme patterns. 