SUMMARYPropagation of the trachoma agent in human cell cultures has made possible the observation by electron microscopy of sequential stages in the replication of the micro-organism. Scattered reticular foci with incomplete limiting membranes were present in the cytoplasm of HeLa cells 12 hr. after exposure to the infectious agent in high multiplicity. After 24 hr. the foci had coalesced to give a single reticular mass. Within it there appeared large and discrete developmental forms from which, by progressive subdivision and diminution in size, !mica1 elementary bodies were formed. The elementary bodies had a prominent limiting membrane, and a dense core within which a fibrillar nucleoid could be recognized. The controversial taxonomic status of the trachoma agent is considered briefly in the light of ultrastructural evidence.

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