Although neuroleptic medications are frequently used to treat the aggressive behavior of persons with mental retardation, little empirical support exists for their efficacy. In the case study reported here, the beta‐blocker propranolol was used to treat the aggressive behavior of a 23‐year‐old woman with severe mental retardation. Initiation of treatment was correlated with a dramatic decrease in aggression and a less pronounced improvement in self‐injurious behavior (SIB). An increase in resting heart‐rate was associated with a significant increase in aggression, and progressive increases in propranolol dosage were correlated with reductions in heart rate and in significant behavioral improvement. Consideration of propranolol as a less intrusive, and possibly more effective, pharmacologic treatment for the aggressive behavior of persons with mental retardation is discussed.