The fine structure of the Malpighian tubule cells of Panstrongylus megistus Burmeister is not very different from that of other blood-sucking insects, specially Triatoma infestans Klug, in terms of types of striated border, distribution of mitochondria, laminated 'concretions', and membranous globules. Details of the tubular fibers which are generally found in the cells of P. megistus are described. The diversified nuclear phenotypes of the distal and proximal cells of the Malpighian tubules appear not to be correlated with specific characteristics of the cytoplasm of the respective cells. Some of the cell nuclei were observed to contain structures resembling laminated 'concretions' closely associated to nucleolar masses. These 'concretions' may represent a step in a viral infectious process rather than result from the inclusion of cytoplasmic matter inside nuclei undergoing fusion.113385394