The paired external glomus of the fully developed pronephros has been studied in early larvae (ammocoetes) of 2 lamprey species, Lampetra fluviatilis and Petromyzon marinus, several weeks after hatching and newly hatched, by use of light-, scanning (SEM) and transmission (TEM) electron microscopy. Three weeks after hatching the glomus is a complex of capillary loops supplied by a single arteriole branching from the aorta. The glomus consists of 3 cell types: podocytes, fenestrated endothelium, and mesangial cells. A basement membrane, which has a close contact to the podocytes, is the only continuous barrier between blood and the coelomic cavity. The glomus exhibits all fine-structural elements known to be essential for function in the glomeruli of other vertebrates. We therefore assume the pronephric glomus of lampreys to be functional in ultrafiltration, with the ultrafiltrate released into the coelomic cavity. In newly hatched larvae, the structure of the glomus is not fully developed. In this earlier stage several afferent arterioles supply each glomus. The endothelial cells in the glomar capillaries still lack regular epithelial organization and resemble mesenchymal cells. However, the presence of typical podocytes stretching over a continuous basement membrane suggests that the tissue is already capable of ultrafiltration.