During early spermatid elongation, at which time germ cells of the mutant l(3)pl (lethal-polyploid) of Drosophila hydei are entirely blocked in their differentiation [16], the microtubular system displays numerous abnormalities on the ultrastructural level. Unstable localization of the otherwise normal centriole, fractionation of the nebenkern, and lack of both nuclear and cellular elongation reflect a general disorganization in cellular arrangement. Moreover, in the growing flagellum, the central tubules are missing. The cytoplasmic microtubules are reduced in number and show various structural abnormalities. Less than one-third have a normal cylindrical cross-section. Single tubules or doublets with one or several side projections of varying lengths are observed, the most frequent representing an α-shape. Cross-bridged tubules may form short chains. The similarity of the observed malformations to the effects of antimitotic drugs suggests that the gene l(3)pl acts at the level of microtubular assembly.