Drought stress is one of the most serious hurdles to the growth and yield of crop plants. Proline plays important roles in lessening adverse effects of an array of environmental stresses, including drought. Thus, in order to investigate the effect of foliar-applied proline on fennel’s response to drought, a 2-year field, along with a pot experiment and an indoor experiment were carried out, where certain local Iranian genotypes were subjected to two levels of irrigation, including control (35-45% depletion of available soil moisture) and drought (75-85% depletion of available soil moisture) and two levels of foliar-applied proline, including control (0 mM) and 20 mM. The field experiment was a 3-replicate split-plot randomized complete block in which irrigation levels were served as main-plots and a factorial of 11 fennel genotypes (namely, Avicenna, Isfahan, Urmia, Bushehr, Birjand, Shiraz, Kerman, Kashan, Mashhad, Hamedan, and Yazd) and two levels of proline were chosen as sub-plots. The pot experiment was a 4-replicate factorial randomized complete block design, encompassing three of the fennel genotypes (namely, Urmia, Shiraz, and Kerman), the two irrigation levels and the two proline levels. In the indoor experiment essential oil components of seed and shoots, fatty acids, and phytosterols of the samples obtained from the pot experiment were determined. Furthermore, antibacterial activity of essential oil of seed and shoots of genotype Shiraz on Staphylococcus aureus was studied. Drought stress led to increases in catalase (CAT) activity, phenolic compounds, proline, soluble carbohydrates, and essential oil concentrations but decreases in concentration of photosynthetic pigments, relative water content )RWC(, leaf water potential (? w ), membrane stability index (MSI), seed yield components and seed yield, harvest index and irrigation water use efficiency (IWUE) for dry mass. Effect of drought on phenolic compounds and proline concentrations, CAT, peroxidase (POX) and ascorbate peroxidase (APX) activities, days to flowering, branches/plant, umbels/plant, seed yield, harvest index, seed WUE, and essential oil yield of field-grown plants differed with genotype. Among yield components examined, seeds/m 2 indicated the greatest correlative associations with seed yield under both control (r 2 =0.93) and drought stress (r 2 =0.92) conditions. Drought led to significant decreases in seed yield of all genotypes; these decreases ranged from 59.3% in Isfahan to 25.7% in Shiraz. Foliar-applied proline affected the concentration of photosynthetic pigments, phenolic compounds, proline, RWC, activities of CAT and POX, MSI, days to flowering, branches/plant and plant height of drought-stressed plants and non-stressed plants differently. It also affected Chl a/b, ? w , RWC, phenolic compounds and proline concentrations, CAT, APX, POX activities, days to flowering, plant height, branches/plant, umbels/plant, dry mass, seed yield, harvest index, IWUE of dry mas, IWUE of seed yield, essential oil concentration and yield in a genotype-specific manner. Foliar-applied proline led to 30.1%, 33.2%, and 35.1% increases in seed yield of Urmia, Birjand, and Yazd, respectively, 17.5% and 21.1% decreases in Avicenna and Isfahan, but did not leave a significant effect on the remaining genotypes in this respect. Drought altered an array of the morpho-physiological traits and seed yield of the pot-grown plants. It led to 32.2%, 19.6%, and 27.5% decreases in root volume, root fresh mass, and root dry mass, respectively, but it led to a 27.2% increase in root length. Effect of drought on RWC, APX activity, plant height, and harvest index of pot-grown plants differed with genotype. Foliar-applied proline affected the MSI, APX activity, umbels/plant, 1000-seed weight, dry mass, seed yield, harvest index, IWUE of dry mass, IWUE of seed yield, and ratio of shoot/root of pot-grown plants in a genotype-specific manner. Despite its genotype-dependent effects on morpho-physiological traits, proline application did not leave notable effects on root attributes of pot-grown plants. External proline resulted in 150 % and 110 % increases in seed weight/plant of Urmia and Shiraz but a 42.5% decrease in that