Xylophagous insects derive nutrients from intractable substrates by producing or ingesting cellulolytic enzymes, or by maintaining associations with symbiotic microbes. Wood-boring cerambycid beetle larvae sometimes house maternally-transmitted endosymbiotic yeasts that are presumed to provide their hosts with nutritional beneÞts. These are thought to be absent from species in the large subfamily Lamiinae; nevertheless yeasts have been repeatedly isolated from the guts of neotropical lamiines. The objective of this study was to conduct transmission electron microscopy (TEM) studies of cerambycid larval midgut tissues to determine if gut yeasts were intracellular, or simply present in the gut lumen. Nine cerambycid larvae were harvested from two trees in the Brazil nut family (Lecythidaceae) in the rain forest of SE Peru; seven were identified using mtDNA sequence data and processed for TEM. Yeasts cultured from larval frass or exuvia, and identified with rDNA sequence data, were identical or similar to yeasts previously isolated from beetles. In TEM analyses yeast cells were found only in the gut lumens, sometimes associated with fragments of thick-walled xylem cells. Apparent bacteriocytes were found in either midgut or fat body tissue of three larval specimens, including two lamiines. This is the Þrst report of a potential fat body symbiosis in a cerambycid beetle. Future studies of cerambycid symbiosis should distinguish the identities and potential roles of free-living organisms in the gut lumen from those of organisms harbored within gut epithelial or fat body tissue.