After this chapter was written a paper on the fine structure of Ichthyosporidium (another species in which cytoplasmic microtubules occur) by Sprague and Vernick (J. Protozool. 21, 667—677. 1974) appeared, providing additional evidence on several microsporidian structures. These investigators were unable to observe any clear association of ribosomes with certain membranes in Ichthyosporidium probably because of inadequate preservation of the membranous material. Therefore, Sprague and Vernick consider the area of flattened sacs near the nucleus as a component of the Golgi apparatus. It should be pointed out, however, that ribosomes associated with similar flattened sacs have been observed in many microsporidian species. Apparently, this region is rough endoplasmic reticulum and not Golgi. Confusion about the nature of reticulum or Golgi) results from the failure of some fixation procedures to reveal the association between membranes and ribosomes.