Triacontanol applied to tomato plants as a foliar spray caused a significant increase in total yield and yield per plant. When triacontantol was added to the growth medium, only a temporary increase in yield and number of fruits was observed. The yield of maize was unaffected by triacontanol, either applied to the leaves or to the growth substrate. These results support an earlier observation that a reduction in photorespiration is involved in the regulatory function of triacontanol, since only the yield of tomato, a C3 plant, was increased. The application method was an important factor in it's effectiveness.