Large squares rarely abscise during the week before anthesis, but young bolls are likely to shed during the week after anthesis. Some change, possibly hormonal, occurs at anthesis that greatly increases the probability that the fruiting form will abscise. Because ABA may promote abscission while IAA inhibits abscission, we measured ABA and IAA contents in squares, flowers, and young bolls. Concentrations of free and amide-linked IAA were very high 6 days before anthesis, decreased to low levels at anthesis, remained low for 4 days, and then free and ester IAA increased. ABA content increased immediately after anthesis. The ratio of ABA to IAA was low before anthesis, increased for 4 days after anthesis, and then decreased as boll abscission rate declined. The high concentration of IAA in squares is probably a major factor in their resistance to shedding. Conversely, the low concentration of IAA in bolls during the week after anthesis is one likely cause of their high probability of shedding. The high concentration of ABA (reported here) and ethylene production (reported earlier) are other likely causes of young boll abscission.