A Heliothis spp. damage submodel (HELDMG) was written for the cotton model GOSSYM. HELDMG simulates the within-plant distribution of damage caused by Heliothis larvae in cotton. This submodel calculates the number of fruit damaged by the Heliothis population and then weights the distribution of the damage within the plant based on larval instar. HELDMG provides a means of integrating a Heliothis model with GOSSYM. The integrated model can be used to study the effects of Heliothis damage on cotton growth and yield.