The relationship between the efficacy of 1% flumethrin in pour-on formulation on the fertility of engorged female Hyalomma dromedarii and exposure time was investigated. Using a contact method, ticks were exposed in vitro to 87 micrograms active ingredient for 1, 5, 10, 30 or 720 min. Fertility inhibition was related to the exposure time. No significant effect was obtained after 1 min exposure, but 5 or 10 min exposure reduced the percentage of females ovipositing, egg mass weight, number of eggs, percentage of females laying eggs that hatch and the conversion efficiency of female weight to egg mass weight. The action of flumethrin was highly pronounced after 30 min; out of ten females, only one oviposited on the 19th day after exposure and the eggs did not hatch. Females exposed for 12 h were paralysed and showed complete sterilization. The pre-oviposition and egg incubation periods of females exposed for 1, 5 or 10 min were prolonged due to the action of the drug.