A few years ago we identified and cloned a new protein, septin 3. While most other septins are expressed in many tissues, septin 3 is found only in brain and is enriched in nerve terminals, but its function was not known. Some septins form a component of the neurofibrillary tangles of Alzheimer's disease, one controls sperm motility, and two others are involved in synaptic vesicle recycling. We searched for its protein binding partners and discovered it binds directly to the endocytic machinery itself, primarily via dynamin. Septin 3 strongly inhibited dynamin GTPase activity. When we transfected septin 3 into cells it completely blocked endocytosis. This identifies a new and surprising component of the endocytosis machinery.