In field experiments during two successive seasons (2003-2004 and 2004-2005), the effects of biofertilization on growth, fruit yield, and oil composition of fennel plants were investigated. Application of biofertilizer, which was a mixture of Azotobacter chroococcum, Azospirillum liboferum, and Bacillus megatherium applied with chemical fertilizers (only 50% of the recommended dosage of NPK) increased vegetative growth (plant height, number of branches, and herb fresh and dry weight per plant) compared to chemical fertilizer treatments only. The tallest plants, the highest number of branches per plant, and the highest fresh and dry weights of plants were obtained from the treatment of biofertilizer plus a half dose of chemical fertilizer (357 kg ammonium sulphate + 238 kg calcium super phosphate + 60 kg potassium sulphate ha-1). The lowest fresh and dry weights of plants occurred with the 50% NPK. Also, addition of biofertilizer with the chemical fertilizer increased these characters more than the half dose of chemical fertilizer alone. Total carbohydrates in the dry plant material were influenced by the biofertilizer. The highest values of total carbohydrates were found in the treatment with biofertilizer plus a half dose of nitrogen and phosphorus. Nitrogen, phosphorus, and potassium levels in the plant tissue increased when soil was inoculated by nitrogen-fixing bacteria, phosphate-dissolving bacteria, and a mixture of all strains, respectively. The least amount of N, P and K in the plant tissue occurred with the half dose of chemical fertilizer. Essential oil content in the fennel fruits was increased due to inoculation compared to the half dose of chemical fertilizer. The highest oil yield per plant was observed with the treatment of biofertilizer plus a half dose of nitrogen and phosphorus. The lowest amount of essential oil yield was obtained with the half dose of chemical fertilizer. Oxygenated compounds were increased as a result of using biofertilizer. The highest anethol (trans-1-methoxy-4-(prop-1-enyl)benzeen; C10H12O)) in fennel essential oil occurred with the half dose of N, P, and K and inoculation with Bacillus megatherium.