The dependence of fruit development on fertilization was studied in two clones of Opuntia ficus-indica, Ofer and BSI. Fruits of the clone Ofer bear fully developed seeds, whereas fruits of the clone BSI contain only degenerated seeds and it was suspected that BSI is parthenocarpic. The two clones differed in their pattern of fruit development. The increase in fruit fresh weight in Ofer was a result of both peel and pulp growth, whereas in BSI fruit growth was mainly due to pulp growth and the peel had reached its final weight almost at anthesis. Pulp growth in BSI commenced earlier and was faster than in Ofer, but the final pulp weight in BSI was only 64% of that in Ofer. Seed growth in BSI was limited to the development of semilignified and lignified seed coats, whereas in Ofer 43·5% of the seeds were completely developed, the remainder being similar to those found in BSI. Fruits bearing well-developed seeds were also found in a 'mutant' of BSI that, like Ofer, contained smaller ovules at anthesis than flower of the regular BSI genotype. Germination in vivo and in vitro were similar in BSI and Ofer, but pollen tubes failed to reach the ovules in the regular BSI genotype, while penetrating the ovules in Ofer and the small-ovuled genotype of BSI. Good-quality fruits similar to Ofer fruits in weight but with a higher peel to pulp ratio developed in BSI after flower-sterilization in the spring and in the autumn. In Ofer, sterilized flowers failed to develop fruits in spring and partially set fruit in the autumn; the fruits consisted almost exclusively of peel tissue. It was concluded that BSI is a vegetative parthenocarpic clone, i.e., that pollination is not required for fruit set and development.Copyright 1993, 1999 Academic Press