In Chlamydomonas growing under 24 h light-dark cycles, chloroplast transcription is under circadian clock control, and peaks early in the morning. The peak (but not trough) requires ongoing cytoplasmic translation, as it is sensitive to cycloheximide (CH). The chloroplast transcriptional apparatus in Chlamydomonas is simpler than in land plants, with only one type of RNA polymerase (RNAP, bacterial) and apparently only one sigma factor (RPOD). Core RNAP can be assayed in vitro with a non-sigma factor dependent template, and is sensitive to rifampicin. We developed a membrane-based assay for RNAP activity, and used it to determine that core activity is only weakly affected by pre-treating cells with CH. Moreover, core chloroplast RNAP activity was steady during a 24 h light-dark cycle. Levels of the sigma factor (RPOD) were examined using western blots, and found to fluctuate less than 25 % during light-dark cycles. These data indicate that circadian regulation of chloroplast transcription is distinct from regulation by sulfur availability, which involves significant changes in RPOD levels. The implications of this data for hypotheses that purport to explain the circadian control mechanism are discussed.