The excretory system of the American dog tick Dermacentor variabilis (Say) consists of a pair of Malpighian tubules that end in a blind ampulla and insert into the rectal sac which in turn leads to the anus. Visceral muscle envelopes the tubules. The single layer of epithelial cells in the tubule wall have an ultrastructure characteristic of cells that transport fluids and ions, e.g. an infolded basal plasma membrane, relatively large numbers of mitochondria, and an extensive development of microvilli on the apical luminal surface. Ixodid ticks have a cycle of excretion during their life span as an adult. Most excretory activity occurs during and after feeding. The purpose of this study was to describe the ultrastructure of the Malpighian tubules in an unfed tick, a fed tick and an ovipositing tick, in an attempt to correlate ultrastructure with excretory activity. We subdivided the tubules into distal, middle and proximal regions in which each area was about 1/3 of a tubule. The proximal region was closest to the rectal sac.