Several trials were conducted over 2 years in California to study the response of olive (cv.Manzanillo) to foliar B application.to determine if foliar B application prior to flowering can increase flowering and fruiting in olive, boron as Solubor was applied at four levels to trees not exhibiting any vegetative symptoms of B deficiency. Foliar B application at 246 or 491 mg.L-1 significantly decreased the percentage of imperfect flowers and increased fruit set. The beneficial effects of foliar B application varied between years and were greater when fruit set was low. Isotopically enriched 10B was applied to determine B mobility during different stages of flower and fruit development, as knowledge of phloem B mobility at various growth stages of plant growth is important for the efficient management of B nutrition. Following foliar 10B application, there was a significant export of the 10B out of the treated leaves, and a significant 10B enrichment in non-treated adjacent organs. These results suggest that there is a specific requirement for B in the olive reproductive process and demonstrate that B can be remobilized from leaves of all ages. The apparent mobility of B at all stages of olive growth, and the translocation of B to developing tissues suggests that foliar B application can probably be effectively used at any stage of olive development.