In the primary spermatocytes of Chironomus pallidivittatus lampbrush chromosomes in pachytene and diplotene stages can be isolated by using a spreading technique.--In the earliest analyzable stages of the pachytene the lampbrush loops, although different in size, are identically structured, consisting of spiralized fibers covered with a thin matrix. These loops contain an average DNA length of 33 mum. They are connected by axial fibers of 6 nm in diameter at intervals of 0.15--0.07 mum without any interposing chromomeres. The axis is double-stranded.--On the basis of the DNA distribution patterns it is supposed that the loops contain those chromatid regions which are included in the bands of the polytene chromosomes of the same species. The lack of a typical RNP transcript in the loops and the finding that 98% of the total DNA is found in the loops lead to the conception that the lampbrush organization in these spermatocytes is primarily involved in the orderly course of crossing-over and in the preparation for the metaphase contraction of the chromatids.