The braconid wasp Microctonus aethiopoides Loan is an idiobiont endoparasitoid of alfalfa weevil adults Hypera postica (Gyllenhal). After oviposition and subsequent egg maturation, large trophic cells called teratocytes dissociate from the serosa and are released into the host hemocoel. These teratocytes are present in large numbers and are visible to the naked eye. It is thought that they accumulate host hemocoelic metabolites for later consumption by the parasitoid larvae. We have undertaken a microscopic study of these gargantuan and complex cells at approximately seven months after parasitization. Parasitized adult weevils were dissected into medium and teratocytes were fixed, embedded, and sectioned at 1 μm. Teratocytes were stained with various specific fluorescent dyes for plasma membrane, Golgi, nuclei, lysosomes, mitochondria, and endoplasmic reticulum (ER). The surface of each cell is covered with a dense microvillar layer. Analysis of fluorescent images showed that these cells do not have condensed nuclei. ER was abundant around the nuclear envelope. Lysosomes were positioned around the periphery of the nucleus and the Golgi apparatus was significantly enlarged, being located around the nuclear envelope.