Abstract: A seal for the accessory port of a medical instrument comprises a first region having an aperture formed in elastomeric material. The aperture permits an accessory of slightly greater diameter to be sealably introduced through the accessory port of the instrument. The seal includes a second region of elastomeric material distal to the first region, which includes a hatch with a closeable passageway. The hatch sealably engages the accessory and also seals the accessory port when no accessory is in place. The first region and/or the second can be stretched and distorted as an accessory passes therethrough. Therefore, it is preferable to dispose a relief cavity between the aperture and the hatch. The relief cavity separates the two regions so that the aperture and hatch can each independently maintain an effective seal around an accessory. The relief cavity also provides a break between the aperture and hatch that minimizes drag on the accessory.