Cotton boll susceptibility to attack by adult boll weevils and cotton bollworms was investigated. Boll age, as defined by heat unit accumulation, was negatively correlated with the number of bolls successfully penetrated by the insect. Individual insects were caged on a single boll and removed after three days. The lignified endocarp appears to be the site of resistance to insect penetration. Boll weevil penetrations of the epicarp and mesocarp remained fairly constant over a 200-500 heat unit range. However, successful penetration of the endocarp declined with increasing heat unit accumulation. Cotton bollworm attempts at and successful penetration of the boll decreased linearly with increasing heat unit accumulation. This relationship between heat unit accumulation and boll susceptibility can be integrated into an end-of-season management program to define the time at which a late-season insecticide application is no longer economically advantageous.