Mouse models are increasingly important for studying human GI pathology. OCT provides minimally invasive, cross-sectional images that indicate the thickness and scattering density of underlying tissue. We have developed endoscopic ultrahigh resolution OCT (UHR-OCT) for the purpose of in vivo imaging in mouse colon. The reduced scale of the mouse colon makes tissue light penetration much less problematic, and high resolution acutely necessary. Higher lateral resolution requires a departure from the traditional cemented GRIN lens design. We support the need for better chromatic aberration than can be achieved by a GRIN lens using commercial raytracing software. We have designed and built a 2mm diameter endoscopic UHR-OCT system achromatized for 770-1020nm for use with a Titanium:sapphire laser with 260 nm bandwidth at full-width-half-maximum centered at 800 nm while achieving a 4.4um lateral spot dimension at focus. A pair of KZFSN5/SFPL53 doublets provides excellent primary and secondary color correction to maintain wide bandwidth through the imaging depth. A slight deviation from normal beam exit angle suppresses collection of the strong back reflection at the exit window surface. The novel design endoscope was built and characterized for through focus bandwidth, axial resolution, signal to noise, and lateral spot dimension. Performance is demonstrated on a variety of ex vivo tissues and in situ mouse colon. Ultrahigh-resolution images of mouse tissue enable the visualization of microscopic features, including crypts that have previously been observed with standard resolution OCT in humans but were too small to see in mouse tissue. Resolution near the cellular level is potentially capable of identifying abnormal crypt formation and dysplastic cellular organization.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.