Electron microscopy has demonstrated a high degree of structural complexity in the body wall of the parasitic nema-tode, Nippostrongylus brasiliensis. The body wall of adult worms is composed of the three layers, the cuticle, hypodermis, and muscle layer. The cuticle is noncellular and has about 14 prominent longitudinal ridges. The cortex layer of the cuticle is composed of homogeneous material and is bounded on the outer side by a three-layered membrane having an inner component 7 m[mu] thick and an outer component 17 m[mu] thick, the two being separated by a lighter zone 10 m[mu] in thickness. The matrix is amorphous, weakly PAS-posltive, an contains embedded transverse bands of dense, strongly PAS-positive material spaced at intervals varying between 1 and 2 [mu], depending upon the body region. In the matrix are also found clusters of protein fibers showing a periodicity of 54 [mu]. The innermost layer of the cuticle[long dash]the fiber layer, is made up of three layers an outer series of transverse bundles probably of collagens, and two inner layers arranged obliquely at an angle of 130[degree] to one anohter. The hypodermis is a single cytoplasmic layer and highly basophilic. It bulges into the body cavity at four places to form the chords. The muscle layer beneath the hypodermis consists of rows of large elongated cells. Each cell has a distal fibrillar zone containing the myofilaments and a proximal protoplasmic zone containing large branching mitochondria and the nucleus. Prominent, electron dense keel-like structures extend from the peripheral sarcolemma into the main body of the cell dividing its distal surface longitudinally into five to eight lobes. Each keel-like structure overlaps the adjacent keel in a shingle-like fashion which could provide contact for impulse propagation and allow the sliding of noncontractile elemnts during contraction. Myofilaments appear to be of two types, 9 m[mu] and 24 m[mu] in diameter. The protoplasmic zone is rich in glycogen. In filariform larvae of N. brasiliensis only two layers, the cortex and the matrix, were observed in the cuticle. The cortex is bordered by a membrane Jid is homogeneous in appearance. The matrix is amorphous and contains transverse bands. The inner layer of the matrix is modified into a crystalloid structure showing regularly spaced parallel lines 180 A apart in longitudinal and 270 A apart in the transverse plane. Below the cuticle is a single cellular hypodermis layer. The muscle layer of the larva beneath the hypodermis has the same general characteristics as that of the adult.