This chapter deals exclusively with studies of winter flounder intestine. Other teleosts may express different transport properties in their intestines and no claim is made that the properties shown here are universal or even necessarily typical. The intestine of the winter flounder, Pseudopleuronectes americanus, has a homogeneous epithelium of absorptive cells without the crypt-villus distribution seen in the intestines of warm-blooded animals. Occasional mucus-bearing cells, basal cells, and endocrine cells can be found, but these are far less numerous than goblet cells or enterchromatiin cells in mammalian or avian intestine. Morphologically and functionally, flounder intestine differs only minimally along its length, which contrasts sharply with mammalian intestine and mammalian renal tubules. Unless a section of tissue is taken very near the diverticula of the stomach, the entire length of the intestine exhibits a transepithelial electric potential difference (PD) between 3 and 6 mV, serosa negative.