Scanning and transmission electron microscopy were used to elucidate the external features and to detail the ultrastructural characteristics of the various tissue systems of Aceria tulipae (Keifer). The integument, alimentary system, and hemolymph are similar to those reported for higher arthropods, although the circulatory system did not appear to have cardiac muscle or aortic systems present. The reproductive system is composed of bilateral ovarioles, oviducts, and a single genital canal. The nervous system is composed of a single neurosynganglion with few lateral processes evident. The gut epithelial linings are only 1 cell layer thick and the organism has no tracheal system. A diagrammatic illustration of the external and internal anatomy is presented.