© 2015 Elsevier B.V. The objective of this study was to evaluate the growth of onion bulbs and the content of flavonoids and phenolic compounds, when the onion plants had been inoculated with Trichoderma asperellum and grown with different doses of mineral fertilizer. Onion seeds of varieties Red Satan and Crystal White were inoculated with one of three T. asperellum isolates from Allium cepa L. (onion), Solanum lycopersicum L. (tomato) and Mangifera indica L. (mango) respectively, and the commercial product, PHC T-22®, based on Trichoderma harzianum, for comparison. Plants were grown in a greenhouse and 100% fertilizer doses applied; after 150 days, bulb mass and the content of phenolic compounds and flavonoids were measured. For all isolates, bulb mass and the content of phenolic compounds and flavonoids increased compared with uninoculated controls but the level of increase was dependent on onion variety and Trichoderma isolate. The T. asperellum isolate from onion, (To), had the greatest capacity for growth promotion, production of indole acetic acid (IAA) and induction of phenolic compounds and flavonoids. Onion plants of the two varieties were then inoculated with the To isolate and grown with 0, 25, 50, 75 and 100% fertilizer doses. The fertilizer requirements to obtain the maximum bulb mass was reduced by up to 50% when plants were inoculated with the To isolate compared to uninoculated plants. At all fertilizer doses, the content of phenolic compounds and flavonoids was higher in the bulbs of plants that had been inoculated with the To isolate, compared with uninoculated control plants. Fertilizer requirements to achieve the highest quantities of both compounds were also reduced by between 50 and 100% in inoculated plants. These data suggest that nutrient availability in onion plants was increased by inoculation with T. asperellum isolate To which modulated the synthesis of phenolic compounds and flavonoids without detrimental effects on the mass of onion bulbs.