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The Gemini Multiobject Spectrograph (GMOS) installed on the Gemini-North telescope has a facility for integral field spectroscopy over the wavelength range 0.4-1.0um. GMOS is converted to this mode by the remote insertion of an integral field unit (IFU) into the beam in place of the masks used for multiobject spectroscopy. With the IFU deployed, integral field spectroscopy is available over a fully-filled contiguous field of 5 x 7 arcsec with a sampling of 0.2 arcsec. A separate field of half the area, but otherwise identical, is also provided to aid background subtraction. The IFU contains 1500 lenslet-coupled fibres and is the largest-format fibre-based IFU yet tested on the sky and the first facility of any type for integral field spectroscopy employed on an 8/10m telescope. We describe the IFU and present results from commissioning.