Ethylene-induced formation of isocoumarin was characterized in relation to ethylene-enhanced respiration in whole or cut carrots (Daucus carota L.). Ethylene concentrations (0.1 to 5 ppm) and temperatures (1 to 15C) that increased respiration also favored a more rapid formation of isocoumarin (8-hydroxy-3-methyl-6-methoxy-3,4-dihydro-isocoumarin). Exposing mature carrots to 0.5 ppm C2H4 for 14 days at 1 or 5C resulted in isocoumarin contents of 20 and 40 mg/100 g peel, respectively. These levels were easily detected as a bitter flavor in the intact carrot roots. Immature carrots formed higher levels of isocoumarin than mature carrots; 180 mg/100 g peel were detected in young carrots stored 14 days at 5C in air containing 0.5 ppm C2H4. Freshly harvested carrots exposed to 5 ppm C2H4 accumulated 4-fold higher isocoumarin levels than those formed by carrots stored 30 days at 5C before exposure to C2H4. An atmosphere of 100% O2 potentiated the effect of C2H4 on isocoumarin formation, resulting in a 5-fold increase over that found in carrots treated with C2H4 in air. A storage atmosphere of 0.5 ppm C2H4 in 1% O2 resulted in isocoumarin levels about one-half those attained in 0.5 ppm C2H4 in air. Sliced, cut, or dropped carrots exposed to C2H4 showed greater isocoumarin accumulation rates than intact uninjured carrots. Peeled baby carrots, however, had little capacity to form isocoumarin. In general, the more rapid the respiratory rise in response to C2H4, the more rapidly isocoumarin accumulated. The greater the respiratory response to ethylene, the higher the level of isocoumarin formed.