A laboratory study was conducted to determine the effect of three extraction parameters (soaking time, extraction time and the ratio of solid to liquid) on the yield and chemical composition of Foeniculum vulgare seeds essential oils. The bioactivity of the essential oil extracted for the optimum extraction parameters was assessed against Culex pipiens mosquito. F. vulgare essential oil composition included large amounts of phenylpropanoids. Through an extraction time of 6h and a ratio solid to liquid of 300g/L we can get over than 72% of trans-anethol without soaking the seeds. With bioassays, essential oils showed different activities on C. pipiens larvae and pupae. Results show that a concentration at 40mg/L was sufficient to register 50% mortality for the second instars larvae and this, after 2h exposition time. Moreover, concentration at 60mg/L ensured after 4h exposition time 90% mortality for the fourth instars larvae. However, pupae needed 24h exposition time to show promising mortalities when using concentration at 200mg/L. Even if laboratory bioassays are only the first step towards the use of essential oils in practical applications, these substances represent a potential alternative to chemical insecticides in some markets.