This study was conducted to determine the influence of l-tryptophan (l-TRP) applied to soil on the yield of two watermelon (Citrullus Lanatus L.) and one muskmelon (Cucumis melo L.) cultivars. Experiments were established in which l-TRP was applied to seedlings as a soil drench in the glasshouse ranging from 6 × 10-5 to 60 mg·kg-1 soil; the seedlings were transplanted into the field 2 weeks later. The application of l-TRP enhanced the cumulative weight of the melons 58% ('Royal Sweet' watermelon), 80% (`Royal Windsor' watermelon), and 42% (`Top Score' muskmelon) over their respective controls: the average weight per fruit was increased up to 43% for watermelon and 36% for muskmelon. No increase in yield was observed when l-TRP was applied to the seedlings directly in the field, but only when added in contained systems 2 weeks before transplanting into the field. The increase in yield was due to a physiological response rather `than a nutritional effect because of the low l-TRP concentrations applied.