Six onchocerca-nodules from five Liberian patients were examined by electron microscopy. The bulk of cells in the centre of the nodules consists of lymphocytes and macrophages. The lymphocytes can be observed in various stages of differentiation. A major part of the macrophages shows degenerative changes with decrease of lysosomes and increase of fatty vacuoles. Near the worms epithelioid cells and giant cells can be found. Other cells in the nodule include polymorphonuclear neutrophils, eosinophils, plasma cells, and mast cells. Adult Onchocerca volvulus show the basic nematode body plan. The cuticle is subdivided into cortex and two layers with differently arranged fibrillar structures. The characteristics of the hypodermal cells are best seen in the lateral chords, in the interchordal regions the hypodermis is flattened by a muscle layer. The intestine typically contains pigment granules, the cells of the epithelium have folds of their cell membranes forming a basal labyrinth. The uterus consists of two tubes in which the development of the microfilariae from the early embryonic forms to the mature stages is examined.