The author presents a clinical vignette of a borderline patient who acts out by having a love affair in order to preserve her identity. His thesis is that one factor in acting out is to serve as a defense against loss of identity. Both loss of identity and acting out may have an origin in the separation-individuation phase in infancy. The acting out may express the child's original attempt to separate from mother by moving away from her in addition to the gratification afforded in mastering the task of motility. The patient described feared losing her identity by being devoured and fused with her husband (mother), a narcissistic submission to her husband based on earlier childhood masochistic and identity conflicts. By having an affair, she was able to actively defend herself against the fear of her incorporative fantasies.