Two overlapping genomic fragments have been cloned from Chlamydia trachomatis serovar L1 DNA. Sequence determination of 2530 bp has revealed two open reading frames coding for 'cysteine-rich' (Cr) proteins. One of these proteins was confirmed, by analysis of the inferred amino acid sequence, as the 60-kDa Cr outer membrane protein associated with differentiation of reticulate bodies (RBs) into elementary bodies (EBs). The other smaller 15-kDa protein contained a high percentage of methionine and cysteine and may correspond to a reported smaller and co-ordinately synthesised Cr outer-membrane protein also associated with RB to EB differentiation. Sequencing showed three potential stem-loop structures within the 5', 3' and intergenic regions of the cloned fragment. Southern-blot analysis revealed that the cloned fragment is conserved in ten serovars of C. trachomatis and that a strongly cross-hybridising fragment is also present in Chlamydia psittaci.