Summary—The ultrastructure of theDrosophila subobscurachromosome regions around the breakpoints of the complex E1 + 2 + 9 + 12gene arrangement was analyzed. This overlapping inversion is formed by the association of the E1, E2, E9, and E12simple inversions. Ultrastructure of sections involving 58D/59A, 61C/D, 62D/63A, 64B/C, 67A/B, and 68B/C breakpoints on Estchromosomes were compared with the ultrastructure of sections involving 58D/68B, 62D/64C, 59A/63A, 64B/68C, 67B/61C, and 67A/61B breakpoints on E1 + 2 + 9 + 12chromosomes. No detectable changes of structural organization on banding patterns induced by the E1 + 2 + 9 + 12inversion were found. Ultrastructural analysis of the two E12breakpoints has, however, facilitated the analysis of the left boundary of E12inversion. Accordingly, we propose 61B/C as a new breakpoint instead of 61C/