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The morphogenesis ofMelolonthapox-like virus was studied by electron microscopy on ultrathin sections of infected adipose cells. The development of the virus occured in virogenic areas, in the vicinity of the nucleus, where different types of immature virus particles could be identified: crescent-shaped particles, spherical particles with a low electron opacity or filled with a dense material. All these virus particles were limited by two membranes, the inner one showing a “unit membrane” structure. Ovalshaped mature particles dispersed in the cytoplasm showed a complex structure. They possess a concave core made of a dense material with a thready structure delimited by a three-layered membrane. An external coat of irregular thickness covers them. An electron dense substance occupies the space between the viral envelopes and the core, in the concavity of which it forms a “lateral body”. The occlusion of mature particles in paracrystalline inclusions is characteristic of this affection. Most of the developmental stages observed resembled those previously described in the pox group. This resemblance is discussed and an hypothetical scheme of the maturation of the virus is proposed.