Biofertilizers, especially Plant Growth Promoting Rhizobacteria (PGPR), are the most reliable replacements for the chemical ones in sustainable agroecosystems nutrient management. To study the effect of Azotobacter, Azospirillum, Pseudomonas (strain 187) and their mixture on lettuce growth, this study was conducted in 2010 at the Research Field of Islamic Azad University, Karaj branch, Iran. The experimental design was randomized complete block design with three replications. Results indicated that inoculating seeds with the biofertilizers significantly increased plant height, the number of leaves, biomass, leaf area index and plant yield, compared with the control (no biofertilizer). Generally, Azospirillum was the best treatment studied in this experiment with the highest value of the measured traits. It increased plant biomass by 43.96% compared with the control.
