Chronic anxiety in a female patient was understood to have multiple meanings in relating to the mother through the fantasy of a symbiotic union. The origins of the fantasy are traced to the patient's poor relationship with a preoccupied and unavailable mother. The fantasy underwent several transformations under the influence of subsequent developmental phases and the special role of aggression in its elaboration. This paper illustrates the reciprocal interactions between separation-individuation and psychosexual development and their influence on the development of the self and gender identity, defenses against aggression, development of a sadomasochistic style, and transference-countertransference interactions. The symbiotic fantasy is seen as carrying the imprints of all developmental phases and as having multiple functions.