We have cloned and sequenced the psaA and psaB genes from the unicellular cyanobacterium Synechocystis sp. PCC 6803. These genes are arranged in tandem, are co-transcribed, and are highly homologous to the psaA and psaB genes previously characterized. RNA was isolated from light-grown cells, from cells put in total darkness with and without glucose, and from cells grown under light-activated heterotrophic growth (LAHG) conditions. Quantitation of hybridization to northern blots revealed only a slight decrease in the accumulation of the psaA-psaB transcript in cells grown in complete darkness with glucose and in LAHG cells, relative to light-grown cells. Accumulation of the psbA transcript steadily declines through dark incubation, with a steady-state level in LAHG cells 28% of that in light-grown cells. Transcripts from psbD, psaD, and rbcLS accumulate in cells grown in complete darkness and in LAHG cells to approximately the same levels as in light-grown cells. Photosynthesis gene transcripts in cells grown in the dark without glucose were detected, but were highly degraded. Our data prove that transcripts from photosynthesis genes do accumulate in dark-grown Synechocystis 6803, which may allow for synthesis and assembly of photosystem (PS) I and PS II in the dark.