Using a groEL gene of Synechococcus sp. PCC 7942 as a DNA probe, a 4.8-kilobase pair (kbp) BamHI fragment of chromosomal DNA of Synechocystis sp. PCC 6803 was cloned. Sequencing of 3.25 kbp of the BamHI fragment revealed three open reading frames. The amino acid sequences deduced from the nucleotide sequences of the two open reading frames are identical to those gained from N-terminal sequencing of purified groEL and groES proteins. This finding demonstrates that these two open reading frames correspond to groEL and groES genes of Synechocystis sp. PCC 6803. groEL of Synechocystis sp. PCC 6803 is remarkably homologous to groEL proteins of other organisms. Southern blot analysis indicates that only one groESL operon is present in the genomic DNA of Synechocystis sp. PCC 6803, whereas the existence of at least two copies of groEL-analogous genes are anticipated. The level of the bicistronic, 2.2-kb transcript of groESL operon increased 100-fold within 15 min upon heat stress. A 9-base pair inverted repeat revealed around the groESL promoter might be involved in regulation of the heat shock response.