The small intestine of piglets orally infected with rotavirus was examined by electron microscopy 18, 24, 48 and 60 hours after infection. At 18 and 24 hours after infection columnar epithelial cells covered the villi. Infected epithelial cells tended to be less electron-dense than uninfected cells and were more numerous at 24 hours after infection. Two types of rotavirus particle were seen, usually within dilated cisternae of the RER: non-enveloped particles measuring 50 to 60 nm and enveloped particles measuring 65 to 75 nm. Non-membrane bound viroplasm containing electron-dense cores was encountered outside the cisternae 18 and 24 hours after infection. Tubular structures measuring 44 to 56 nm were often found in nuclei of infected cells. Single-membraned (44 to 55 nm) and double-membraned tubules (70 to 80 nm) associated with viral manufacture were found in the cytoplasm of infected cells. At 48 and 60 hours after infection a proportion of villous epithelial cells were cuboidal. Virus particles were detected in only a few epithelial cells and nuclear and cytoplasmic tubules were not seen. At all times after infection some infected cells showed a reduction in the number and size of the microvilli comprising the brush border.