In an ultrastructural study of the delphacid planthopper, Peregrinus maidis, vector of maize mosaic virus (MMV) and maize stripe virus (MStpV), the following structures were found in several organs of MMV-inoculative, MStpV-inoculative, and noninoculative insects: (a) paramyxovirus-like particles (PLP), (b) filamentous rhabdo-like structures (FRS), (c) spiked double-membrane structures (SDS) that were always associated, and sometimes contiguous, with FRS, and (d) rickettsia-like structures (RLS). In salivary gland acini, the PLP, FRS, and SDS were usually found in extracellular spaces between basal infoldings of plasma membranes, whereas the RLS were intracytoplasmic or intranuclear. The PLP and FRS appeared to bud through plasma membranes. The colony of P. maidis in which these structures were found suffered from retarded development and premature death. Limited trials to isolate a paramyxovirus from this colony were unsuccessful. However, further trials led to the isolation of Staphylococcus sciuri, which proved pathogenic when injected into P. maidis and two cicadellid leafhoppers; Dalbulus maidis and Graminella nigrifrons.