A homozygous self-compatible almond with a high level of inbreeding producing a reasonable yield when cross-pollinated was experimentally found to dr op fruits when self-pollinated. The aim of this work was to elucidate the causes of fruit abortion after selfing by studying pollen tube growth, megagametophyte development and fruit set following controlled self and cross-pollinations. Pollen tubes reached the ovary for both pollinations, however, differences were observed for embryo and endosperm development. Thus, while for cross-pollination a developing embryo and an incipient endosperm were observed, in self-fertiliz ed ovules a degenerating embryo was observed in most cases. Thirty days after pollination, the percentage of fruit set was similar for both pollination types. In contrast, 60 days after self- pollination fruit set was very low or nil. These results indicate that the abnormal development of the megagametophyte in this genotype seems to be an expression of the high level of inbreeding.