An extensional rheometer for creep experiments at relatively high tensile stresses is presented. Elongational viscosities and creep recoveries as functions of time can be measured. The new device is called Windbix Extensional Rheometer. "Windbix" is a neologism reminding of the Dutch word for air-gun ("Wind Buks") with reference to the device's pneumatic mechanism to create the tensile force. The rheometer was validated with a LDPE identical to IUPAC A. Results are presented and partly compared with published data. The new filament stretching rheometer uses a pneumatic (or "expansion") tube for creating a preadjustable tensile Stress Lip to 0.3 MPa, which is constant during extension of the sample. The time needed for the adjustment of this stress is less than 0.02 s. In principle, this fast mechanism enables theological measurements at high strain rates. As a prerequisite the usual rigid end plates are replaced by flexible textile mats, pliably following the contraction of the top of the sample during elongation. In this way, influences of necking are effectively reduced. Isothermal extension takes place in a thermostated glass tube flushed with inert Gas. In principle samples of suitable polymers can be extended from an initial length of 10 mm up to a final length of nearly I m. This corresponds to a total Hencky strain up to 4.6. Originally, this machine was designed for a quantitative evaluation of the influence of extensional flow on the crystallization kinetics of semicrystalline polymers, where a rapid short term extension is indispensable (see Part 11). (C) 2004 The Society of Rheology.