Heat tolerance in 23 tropical and one temperate fruit crops was evaluated by determining chlorophyll fluorescence (the ratio of the variable fluorescence to the maximum fluorescence (F v F m ), and the basal fluorescence (F 0 )). The ratio (R v ) of F v F m in leaves exposed to high temperature (45°C for 20 min) to F v F m to control temperature (25°C for 20 min) was found to be highly and negatively correlated to the ratio (R O ) of F O exposed to the high temperature toF O exposed to the control temperature (r = -0.92 * * ). Leaves sampled in mid-July (3.5 months old) were slightly but significantly more tolerant to heat than 2.5-month-old leaves sampled in early to mid-June. The ratio of the genetic variance to the total variance in the measurements was 0.90 forR v , and 0.89 for R O . Pineapple, coconut palm, and Annona species in Annonaceae (cherimoya, soursop, sugar apple) were heat-tolerant, whereas Syzygium species in Myrtaceae (Java apple and rose apple), longan, and peach were sensitive.