The sequence of the Inhibitor element Spm-I8 isolated from the wx-m8 allele has been determined. The element is 2242 bp in length. Its ends can be folded into long stem and loop structures. In a line containing the autonomous En element (wx-m8+En) we have detected a 2.5-kb transcript hybridizing to Spm-I8. A cDNA copy of this En-specific transcript containing 1.2 kb of the 3' end was cloned and it DNA sequence was determined. The 3' half of the cDNA is homologous to Spm-I8 and the region of homology is interrupted by intervening sequences. In the absence of an autonomous En element two chimeric transcripts are observed in the wx-m8 line which are probably initiated at the wx promoter and terminate in the Spm-I8 insertion. In the presence of the En element, these transcripts are suppressed, possibly by a trans-acting function of En, inhibiting transcription read-through into Spm-I8.