The anatomy of abscissed avocado fruitlets of the Hass variety was compared with that of actively growing control fruitlets to investigate the reason for abscission. Control fruitlets had a small embryo (4–14 cells) and numerous endosperm cells by seven days after pollination. Cotyledon development had commenced by 21 days and a root/shoot axis was present at 28 days. Over 90 per cent of the fruitlets which abscissed during the first week after the end of flowering were unfertilized and 18 per cent were abnormal. By the fourth week after the end of flowering all abscissed fruitlets were fertilized and none abnormal. Abnormalities included ovaries with more than one embryosac or ovule, with an immature embryo-sac, with an ovule in an abnormal position or with a deficiency in ovule structure. In a few fertilized abscissed fruitlets either the embryo or endosperm had ceased development first. In the majority of cases embryo and endosperm were anatomically normal. The majority of fertilized abscissed fruitlets had reached a stage corresponding to 14 days after pollination of the control tree. Degeneration was often observed in both unfertilized and fertilized abscissed fruitlets. It is suggested that this degeneration occurred in the period between the cessation of development and abscission of the fruit. This period was approximately one week. The proportion of abnormal ovaries was too low to significantly affect yields, and sufficient flowers had been fertilized to give an adequate crop. No anatomical reason for the high rate of fruitlet abscission was observed.