AimsTo study the marked resistance of Periplaneta americana to entomopathogenic Metarhizium anisopliae. Methods and ResultsThe low susceptibility of 4th instar nymphs applied topically with conidia seemed to be related to an active removal of conidia by the cockroach and to a disabled or retarded germination and subsequent development of conidia on the cuticle (up to 80% germination in the next 7days after application). Inhibitions or delays of germination were related to the composition of the epicuticular fatty acids (301% w/w oleic, 283% w/w linoleic, 245% w/w palmitic and 117% w/w stearic acid) reported here. Propagules invading the nymphs through the cuticle took at least 3days to reach the haemocoel, and no propagules were found after day 8 post-treatment. Strain IP 46 infected >50% of nymphs treated with doses 2x10(4)hyphal bodies (HB)nymph(-1) and reduced the survival of nymphs 50%. Most nymphs (>70%) survived after injection of 6x10(3) and 2x10(3)HBnymph(-1). ConclusionsFindings emphasize a distinct resistance of nymphs of the American cockroach to infections by M.anisopliae. Significance and Impact of the StudyOur findings provide support for the development of biological control of this synanthropic cockroach pest.