Following binding and internalization into the host cell cytoplasm, elementary bodies (EB) of the obligate intracellular bacterium Chlamydia trachomatis undergo a developmental process resulting in production of reticulate bodies (RB), the vegetative growth form of the organism. EB are metabolically inactive, but EB to RB transformation requires bacterial protein synthesis. Using HeLa cells infected with EB of C. trachomatis serovar C, we examined the time of first appearance of transcripts from several genes whose products are required for assembly and function of the chlamydial protein synthetic system. We monitored appearance of chlamydial RNAs using reverse transcription-polymerase chain reaction assays targeting primary transcripts from the bacterial rRNA operons, and mRNAs encoding the glycyl tRNA synthetase and the ribosomal proteins S5 and L5. Transcripts from the proximal rRNA promoters, and those from the r-protein and tRNA synthetase genes, are detectable as early as 4 h after EB-host binding; transcripts from distal rRNA promoters do not appear until 6 h post-infection. Thus, expression of bacterial genes whose products are required for protein synthesis begins earlier in chlamydial EB to RB development than previously thought.