The complete nucleotide sequence of the mobile genetic element IS30, a resident of Escherichia coli K12, is 1221 bp long. A large open reading frame, preceded by possible transcription and translation control signals, could encode a basic protein of 383 amino acids which might presumably function as transposase. No large in-frame open reading frame is present on the opposite strand. The 26 bp long terminal inverted repeats have some sequence homology with the c-end of the phage Mu genome and with the terminal inverted repeats of the Halobacterium halobium insertion element ISH50. The IS30 sequence has no significant homology with any other sequenced prokaryotic insertion sequences.