The restructuring of basal bodies was analyzed in spermatogenesis of a caddisfly, Potamophylax rotundipennis (Limnephilidae), using electron microscopy of ultrathin serial sections. Flagellar outgrowth was characterized by the belated formation of the central pair of microtubules. Flagellar stubs of metaphase I spermatocytes were devoid of central microtubules. The proximal portion of the basal bodies contained a cartwheel structure. Spermatids showed long flagella with the usual 9 + 2 arrangement of microtubules. Surprisingly, the central pair of flagellar microtubules extended deeply into the lumen of the basal body. This may be rendered possible by the attachment of the basal bodies to the outer face of the nuclear envelope in spermatids. The outer nuclear membrane may nucleate the assembly of the central pair of microtubules in spermatogenesis of the caddisfly. Another unusual feature of basal bodies in the caddisfly was the presence of a cylinder of electron-dense material within their lumen. Its function may lie in linking the microtubular blades of the basal bodies with one another. Finally, an observation within the spermatid tail is remarkable: the accessory tubules consistently possessed a central filament throughout their length. (C) 1995 Academic Press, Inc.