Tropical fruit production often suffers from the disadvantage that the cropping seasons and postharvest life are short. The short production season is related to the tendency of the trees to flower over a short period, presumably in response to some environmental trigger. This study was concerned with the identity of this trigger in avocado, lemon, lychee and mango and with the possibility of using this knowledge to manipulate flowering time. It was found that avocado, lychee, and mango could only be induced to flower by exposure to low temperatures (18/15°C for lychee and mango; 23/18°C for avocado). Water stress could not replace low temperatures, and leaf water potentials were not affected by temperature, indicating that temperature effects were not mediated by water stress. Lemon, on the other hand, flowered almost entirely in response to water stress at temperatures of 18/10°C or higher, although it flowered freely under natural winter conditions (temperature down to 7°C) without water stress. Flowering was slightly delayed when irradiance was reduced to 3-5 MJ m-2 d-1 but otherwise was unaffected. It is suggested that this is consistent with the origins of these species as under-storey trees of the tropical rainforest. Vegetative growth usually varied inversely with flowering, but the correlation was not consistent. Similarly, starch contents tended to be higher in trees which were provided with conditions which induced flowering, but the relationship seems to be coincidental. Nitrogen treatments had no specific effects on flowering. A study of the morphological changes in the apex in mango using the scanning electron microscope showed that floral initiation occurred after four or five weeks at 15°/10°C, but not at a water stress of 90-93% relative water content. It is suggested that the most promising opportunities for the control of flowering time in lychee and mango depend on manipulation of the dormant phase before floral induction.