The Escherichia coli heat-stable enterotoxin STII gene in P307 is flanked by inverted repeat sequences, suggesting that the STII gene is part of a transposon. To study the transposability, a DNA fragment containing the putative STII transposon has been cloned. Results of transposition assays indicated that the STII gene can transpose from one plasmid to another. The size of the transposon has been determined to be approximately 9 kilobases. The structure and the location of the STII gene in clinical isolates of Escherichia coli have been investigated by restriction enzyme analyses. The structural genes of STII from different clinical isolates appear to be uniform in size, but the flanking sequences are heterogeneous. This result suggests that the STII genes in different isolates are not on the same transposon as observed in P307.