Mirazid -a purified oleo-resin extract derived from myrrh- with previously proved trematocidal activity, has been recently reported to be also effective against protozoa like Trichomonas vaginalis and cryptosporidium in humans. This experimental study aimed at investigating the possible therapeutic effect of mirazid on experimental Giardia lamblia infection in albino rats, using tinidazole as a therapeutic control. Results proved the efficacy of mirazid, as indicated by a 100% reduction in parasite- load of both intestinal and fecal parasitic counts, a direct toxic effect on Giardia trophozoite as revealed by SEM and TEM in form of cell membrane erosions together with increased cytoplasmic vaculation and dissociation, and also by reversal of the observed mucosal damage associated with Giardia after complete treatment. Starting trophozoite invasion of the mucosa was also demonstrated by TEM; its mechanism and possible pathogenic effect in giardiasis were discussed.