Ultrathin sections were prepared of viruliferous Agallia constricta Van Duzee, a leafhopper vector and host of the plant pathogenic wound-tumor virus (WTV), in an attempt to localize the virus within tissues and cells of various anthropod organs. Earlier infectivity and serological tests had revealed the presence of virus in some insect organs and tissues, without permitting precise localization. In the present study, excised fat body tissues, gut, Malpighian tubules, and salivary glands, as well as whole abdomens, were fixed and embedded in methacrylate or in epoxy resin. Alternating thick and ultrathin sections were made with a Porter-Blum microtome and stained with uranyl acetate and lead. For light microscopy, the thick sections were stained by Delafield's hematoxylin and eosin. Thin sections, dried on carbon-coated collodion or Formvar film, were examined by a Siemens Elmiskop I. Largest accumulations of WTV, often in the form of microcrystals, were found in the fat body tissues; crystalline arrangements were also seen in muscle cells. Virus clusters were observed in defined cytoplasmic structures within the Malpighian tubules, mycetome, trachea, and epidermis. Only one or two lobes of the salivary gland contained scattered virus particles not associated with defined cytoplasmic structures. The findings point to the fat body as the most likely site of virus multiplication. The occurrence of virus accumulations in almost all organs of A. constricta is interpreted as direct evidence for systemic invasion of this arthropod host. Lack of a sizable accumulation of WTV in the salivary gland may explain the erratic and poor transmission record of the insect vector. This finding is in contrast with findings made with the very efficient vector of rice dwarf virus, in which the lobes of the salivary gland contain large masses of virus particles.