The false stable fly Muscina stabulans (Diptera: Muscidae) is worldwide distributed with medical, veterinary and forensic importance. This study investigated the disruptive effects of Pyriproxyfen on survival, development, metamorphosis and morphogenesis of this fly. A series of Pyriproxyfen (5.0, 1.0, 0.5, 0.1 & 0.01μg/larva) was topically applied onto the early
last (3rd) instar larvae and prepupae. Regardless the time of treatment, different larval, pupal and adult mortalities were recorded. LD50 values of pyriproxyfen were calculated as 0.242 & 0.444μg/stage, respectively. Pyriproxyfen exhibited a shortening action on larval and pupal durations, with an exception of prolongation of pupal duration after treatment of the early last instar larvae. Adult longevity was pronouncedly shortened; and pupation was suppressed regardless the time of treatment. Adult emergence was partially blocked parallel to the dose level or in no certain trend. Various percentages of larval-pupal intermediates were recorded after treatment of early last instar larvae and some pupal-adult intermediates were produced after treatment of prepupae with the higher three doses. After prepupae treatment with lower three doses of pyriproxyfen, some failed to metamorphose into pupae and remained as permanent ones. Some deformed pupae were observed only after treatment of the last instar larvae. Topical application of pyriproxyfen onto the last instar larvae or prepupae caused emergence of some deformed adults. All deformed flies died within few days without mating.