Abstract: Recent studies have revealed multiple dynamic complexes that are precursors to eukaryotic ribosomes. EM visualization of nascent rRNA transcripts provides in vivo temporal and structural context for these events. In exponentially growing S. cerevisiae, pre-18S rRNA is dramatically compacted into a large particle (SSU processome) within seconds of completion of its transcription and is released cotranscriptionally by cleavage in ITS1. After cleavage, a new terminal knob is formed on the nascent large subunit rRNA, compacting it progressively in a 5'-3' direction. Depletion of individual components shows that cotranscriptional SSU processome formation is a sensitive indicator of the occurrence or timing of the early A(0)-A(2) cleavages and depends on factors not isolated in preribosome complexes, as well as on favorable growth conditions. The results show that the approximately 40 components of the SSU processome/90S preribosome can complete their tasks within approximately 85 s in optimal conditions.