Abstract: The boron tolerance of two summer squash cultivars (Cucurbita pepo L ‘Aristocrat Zucchini’ and ‘Peter Pan Scallop’) and one winter squash cultivar (Cucurbita moschata Poir ‘Butter boy’) was determined in large, outdoor sand cultures Boron treatments were imposed by irrigation with culture solutions that contained 10, 30, 60, 90, 120, or 150 mg B L-1 Relative fruit yields of ‘Zucchini’, ‘Scallop’, and ‘Butter boy’ were reduced 52%, 98%, and 43% with each unit (mg L-1) increase in soil solution B (Bsw)>27, 49, and 10 mg B L-1, respectively Reduced yields of all cultivars were attributed to a reduction in fruit number and not fruit size Boron concentrations in leaves and fruit were directly correlated to Bsw