Large inserts have been found to appear within the yellow (y) locus of several unstable mutant lines derived from a parental strain (y2 sc+ w(aG)) of Drosophila melanogaster. The inserts of the individual mutant lines correspond roughly to a series of multimers (4.1, 7.1, 14.3 and 28.9 kbp, respectively) and are always located at the same position, somewhat upstream of the y gene transcription start site. Each of them begins and ends with a 1-kbp sequence which shares restriction sites with P elements. One of these inserts (14.3 kbp) has been partially sequenced (1270 bp from the 5' end). The first 1185 bp match exactly the sequence of an internally deleted P element. The remaining part of the sequence, on the other hand, is unrelated to P elements. It is assumed that large DNA sequences may be captured by the flanking P-element fragments and inserted into new chromosomal sites.